feat!: remove deprecated functionality and drop PHP unsupported version compatibility

BREAKING CHANGE: 

This PR has been automatically generated by fountain. Please review it and merge it if everything is fine.

> [!CAUTION]
> BREAKING CHANGE.
> The SDK supports PHP version 8.0 and newer. Versions lower than 8.0 are not supported any longer!

## Address deprecated functionality warnings:

<details><summary>

### 1. Set jsonSerialize() return type explicitly

</summary>

* Since [8.2](https://github.com/php/php-src/blob/PHP-8.1.0/ext/json/json.stub.php#L18) - `JsonSerializable` requires the implementations to explicitly return the returned type.

```
Deprecated: Return type of ProcessOut\Invoice::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```

</details>


<details><summary>

### 2. Remove curl_close() invocations - it's being done automatically during GC now.

</summary>

* Since [8.0](https://php.watch/versions/8.0/resource-CurlHandle) - `curl_close()` has no effect and is deprecated since [8.5](https://php.watch/versions/8.5/curl_close-curl_share_close-deprecated).

```
Deprecated: Function curl_close() is deprecated since 8.5, as it has no effect since PHP 8.0
```

</details>

<details><summary>

### 3. Add "?" explicit nullable type operator when applicable.

</summary>

* Since [8.0](https://php.watch/versions/8.0/null-safe-operator) - added support for "nullable" types - nullability should be explicitly expressed using the `?` operator.

```
Deprecated: ProcessOut\Exceptions\AuthenticationException::__construct(): Implicitly marking parameter $previous as nullable is deprecated, the explicit nullable type must be used instead
```

</details>

## Remove non-existent endpoints and related resources
* /activities
* /coupons
* /customers/:id/subscriptions
* /plans
* /subscriptions
* /supervised-projects

Author: tomasz-pazdziurek-cko

README.md

@@ -9,7 +9,7 @@ This package provides bindings to the ProcessOut API.
 Dependencies
 ------------
 
-* PHP 5.4 or above
+* PHP 8.0 or above
 * cURL PHP extension
 
 Installation
@@ -20,7 +20,7 @@ The package's installation is done using composer. Simply add these lines to you
 ```json
 {
     "require": {
-        "processout/processout-php": "^7.4.0"
+        "processout/processout-php": "^8.0.0"
     }
 }
 ```
@@ -41,7 +41,7 @@ cloning the directory into your project
 git clone https://github.com/processout/processout-php
 ```
 
-or downloading the latest release at 
+or downloading the latest release at
 [github.com/ProcessOut/processout-php/releases](https://github.com/ProcessOut/processout-php/releases)
 and including the ProcessOut init file
 

composer.json

@@ -15,7 +15,7 @@
     }
   ],
   "require": {
-    "php": ">=5.4",
+    "php": ">=8.0",
     "ext-curl": "*",
     "ext-mbstring": "*"
   },

init.php

@@ -12,8 +12,6 @@
 include_once(dirname(__FILE__) . "/src/Networking/Request.php");
 include_once(dirname(__FILE__) . "/src/Networking/Response.php");
 
-include_once(dirname(__FILE__) . "/src/Activity.php");
-include_once(dirname(__FILE__) . "/src/Addon.php");
 include_once(dirname(__FILE__) . "/src/APIVersion.php");
 include_once(dirname(__FILE__) . "/src/ApplePayAlternativeMerchantCertificates.php");
 include_once(dirname(__FILE__) . "/src/AlternativeMerchantCertificate.php");
@@ -23,11 +21,9 @@
 include_once(dirname(__FILE__) . "/src/Card.php");
 include_once(dirname(__FILE__) . "/src/CardInformation.php");
 include_once(dirname(__FILE__) . "/src/Phone.php");
-include_once(dirname(__FILE__) . "/src/Coupon.php");
 include_once(dirname(__FILE__) . "/src/Customer.php");
 include_once(dirname(__FILE__) . "/src/CustomerPhone.php");
 include_once(dirname(__FILE__) . "/src/Token.php");
-include_once(dirname(__FILE__) . "/src/Discount.php");
 include_once(dirname(__FILE__) . "/src/Event.php");
 include_once(dirname(__FILE__) . "/src/ExportLayout.php");
 include_once(dirname(__FILE__) . "/src/ExportLayoutConfiguration.php");
@@ -59,13 +55,11 @@
 include_once(dirname(__FILE__) . "/src/Payout.php");
 include_once(dirname(__FILE__) . "/src/PayoutItem.php");
 include_once(dirname(__FILE__) . "/src/PayoutItemAmountBreakdowns.php");
-include_once(dirname(__FILE__) . "/src/Plan.php");
 include_once(dirname(__FILE__) . "/src/Product.php");
 include_once(dirname(__FILE__) . "/src/Project.php");
 include_once(dirname(__FILE__) . "/src/ProjectSFTPSettings.php");
 include_once(dirname(__FILE__) . "/src/ProjectSFTPSettingsPublic.php");
 include_once(dirname(__FILE__) . "/src/Refund.php");
-include_once(dirname(__FILE__) . "/src/Subscription.php");
 include_once(dirname(__FILE__) . "/src/Transaction.php");
 include_once(dirname(__FILE__) . "/src/NativeAPMResponse.php");
 include_once(dirname(__FILE__) . "/src/NativeAPMParameterDefinition.php");
@@ -77,11 +71,11 @@
 include_once(dirname(__FILE__) . "/src/TransactionOperation.php");
 include_once(dirname(__FILE__) . "/src/Webhook.php");
 include_once(dirname(__FILE__) . "/src/WebhookEndpoint.php");
+include_once(dirname(__FILE__) . "/src/CardUpdateRequest.php");
+include_once(dirname(__FILE__) . "/src/CardCreateRequest.php");
 include_once(dirname(__FILE__) . "/src/Device.php");
 include_once(dirname(__FILE__) . "/src/CardContact.php");
 include_once(dirname(__FILE__) . "/src/CardShipping.php");
-include_once(dirname(__FILE__) . "/src/CardUpdateRequest.php");
-include_once(dirname(__FILE__) . "/src/CardCreateRequest.php");
 include_once(dirname(__FILE__) . "/src/ErrorCodes.php");
 include_once(dirname(__FILE__) . "/src/CategoryErrorCodes.php");
 include_once(dirname(__FILE__) . "/src/ExternalThreeDS.php");

src/APIVersion.php

@@ -135,9 +135,9 @@ public function fillWithData($data)
 
     /**
      * Implements the JsonSerializable interface
-     * @return object
+     * @return array
      */
-    public function jsonSerialize() {
+    public function jsonSerialize(): array {
         return array(
             "name" => $this->getName(),
             "description" => $this->getDescription(),