Dimensions Support: Switch dimensions block support back to using spacing key

[aaronrobertshaw]

Description

This reverts the registration of dimensions block support back to using the spacing key. This is to ensure that it still aligns with the previous registration of block support within WordPress 5.8.

How has this been tested?

• Ran npm run wp-env run phpunit 'phpunit -c /var/www/html/wp-content/plugins/gutenberg/phpunit.xml.dist --verbose --filter stylesheet /var/www/html/wp-content/plugins/gutenberg/phpunit/class-wp-theme-json-test.php'
• Ran npm run test-unit:watch packages/block-editor/src/hooks/test/style.js
• Added padding via block supports to dynamic block e.g. site-tagline

Types of changes

Bug 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).