Use built-in core object in github-script example

[ryohighbridge]

Replace manual require of @actions/core with the built-in core object that is automatically available in github-script action. This provides a cleaner and more straightforward approach.

Reference: https://github.com/actions/github-script?tab=readme-ov-file#this-action

Why:

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
actions/how-tos/secure-your-work/security-harden-deployments/oidc-in-cloud-providers.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

This pull request simplifies a github-script example in the OIDC documentation by removing the manual require of @actions/core and using the built-in core object that is automatically available in the github-script action environment.

Changes:

• Removed the manual require('@actions/core') statement and the coredemo variable
• Updated code to use the built-in core object directly for getIDToken() and setOutput() methods