Add missing field "forum_discussion"

[voegii]

🔀 Purpose of this PR:

• [x ] 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:

The field "forum_discussion" is not created during installation. It is only created when updating from a version lower than 2026012002.

This PR adds the field "forum_discussion" to db/install.xml. Additionally, it creates a new plugin version with a new update savepoint which creates the field for all fresh installations after 2026012002.

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📋 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.
• [x ] 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.
• [x ] 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 Missing field "forum_discussion" in install.xml #320