Update Version / Integrate some open PRs as starting point

[lenaherf]

Note: Please fill out all relevant sections and remove irrelevant ones.

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

• Migrated CI testing to the shared Learnweb Moodle workflow.
• Updated the action-menu implementation and the thumbs-down icon component.
• Addressed Moodle CodeChecker, PHPDoc, validation, Mustache lint, and other code-quality findings.
• Adding changelog.

This PR establishes a baseline for maintaining the Scheduler plugin across current Moodle releases. Further (compatibility) issues can be handled in follow-up changes.

📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that
  should not appear on the productive branch.
• Code only uses language strings instead of hard-coded strings.
• If there are changes in the database: I updated/created the necessary upgrade steps in db/upgrade.php and
  updated the version.php.
• If there are changes in javascript: I build new .min files with the grunt amd command.
• If it is a Moodle update PR: I read the release notes, updated the version.php and the CHANGES.md.
  I ran all tests thoroughly checking for errors. I checked if bootstrap had any changes/deprecations that require
  changes in the plugins UI.

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🔍 Related Issues

• Related to PRs: Coding style and Bootstrap 5 compatibility, resolves #77 #68 , Thumbs down icon as font awesome. #69, Use Font Awesome icon for thumb down button, too. #78, Respect coding style and ensure Bootstrap 5 compatibility #77

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🧾📸🌐 Additional Information (like screenshots, documentation, links, etc.)

Any other relevant information.

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