Update version required before gutenberg_safe_style_attrs filter can be removed

[aaronrobertshaw]

Description

Changes made to the gutenberg_safe_style_attrs filter in #31641 and #36280 changed the minimum version requirement before the filter could be removed to be 5.9. This PR just updates the comment to reflect this requirement.

Testing Instructions

Double-check the WP version that contains the changes from #31641 and #36280.

Types of changes

Typo fix.

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.

[ramonjd]

Double-check the WP version that contains the changes from #31641 and #36280.

#31641

Changes made it into 5.9 (not 5.8)

#36280

Changes are visible in 5.9 but not 5.8 (the filter entry for example)

LGTM