Group: Update block supports.

[ndiego]

Description

Margin support for Group blocks was added in #37344. This PR updates the core block documentation to reflect this.

Types of changes

Updated docs.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[Mamaduka]

Thanks for the quick follow-up, @ndiego.