Fix gutenberg/gutenberg-coding-standards licensing issues

[anton-vlasenko]

What?

This PR:

1. removes third-party code from the gutenberg/gutenberg-coding-standards (GBCS) package and removes unused code;
2. switches GBCS to the GPL-2.0-or-later license as it provides more freedoms and access to licensees than MIT-license code.

Why?

Ensure the package does not use any third-party code.

How?

• change the license to GPL-2.0-or-later;
• Remove third-party code from the SinceTagSniff::find_docblock() method;
• Remove third-party code from the SinceTagSniff::is_function_call() method;
• Remove the SinceTagSniff::find_hook_docblock() method and add the new find_previous_line_token() method (refactoring);
• Fix typo in the SinceTagSniff::process() method;
• Remove unused code from bootstrap.php and make variables use snake case;

Testing Instructions

1. Review the code changes and ensure that all CI jobs pass successfully.
2. Navigate to test/php/gutenberg-coding-standards/.
3. Run composer update.
4. Run composer run check-all.
5. Ensure there are no errors in the console.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

Flaky tests detected in 2716212.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/9214390920
📝 Reported issues:

• [Flaky Test] Clicking on inserter while on zoom-out should open the patterns tab on the inserter #61806 in /test/e2e/specs/site-editor/zoom-out.spec.js

[github-actions]

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Co-authored-by: anton-vlasenko <antonvlasenko@git.wordpress.org>

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