Bugfix: letter group does not expands with "show more" button

[TamaroWalter]

When clicking the show more button of a post letter, the letter group did not expand with it. Now it does by adjusting its maxHeight attribute after the post letter expanded

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:

A bug was recently discovered.

The bug: When clicking on the "show" more button of a post, the letter expanded but the letter group did not. the letter then overflowed the letter group.

The solution: Adjust the maxHeight of the letter group after a clicking the "show more" button

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📋 Checklist

Please confirm the following (check all that apply):

• 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.
• If there are changes in javascript: I build new .min files with the grunt amd command.