Revisiting the GitHub Actions Release system (.flatpak + .deb)

[Stefterv]

Out of caution we kept the Ant GitHub Actions in the repository, this PR will remove them and introduce a revised version of the Gradle based GitHub Actions, providing the following.

• Setup specific to Processing is now stored in a shared action, this will cut down on repeated configuration, if anyone knows a good way to clean up the matrices I would like to know.
• This PR pulls the configuration files out of Gradle and places them into app/linux similar to macOS and Windows.
• This PR separates the different platforms in the release action, originally they were done with a matrix but it started to have too many exceptions and platform specific flow that I decided to have a unique job for every platform still with a matrix on the architectures.
• This PR adds support for flatpak (not yet flathub) and add the .deb file to the release which is used by flatpak and snap.

Closes #890
Closes #1211
and includes #1202

TODO

• Include the new SVG-based icons for every platform (Should be a new issue)
  - Separate out Windows signing
  - Separate out macOS notarisation

Follow up tasks

• Separate out the macOS notarisation step for a future macOS App Store Release
• Flathub Submission

[GoToLoop]

Maybe also add the AppImage universal format?

[kaechele]

This is not how --install-deps-from works.

You'll need to

    commandLine(
        "flatpak",
        "remote-add",
        "--user",
        "--if-not-exists",
        "flathub",
        "https://flathub.org/repo/flathub.flatpakrepo"
    )

first and then --install-deps-from=flathub.

[kaechele]

25.08 is out at this point.

[tristanbay]

Bumping this because I also want an up-to-date Flatpak package (I try to avoid using Snap)

[catilac]

approved! merge when ready @Stefterv