Block Supports: strip custom CSS from blocks for users without edit_css capability

Adds capability-gated CSS stripping so that when a user without `edit_css` saves a post, any `style.css` block attributes are removed from block comments using `WP_Block_Parser::next_token()`.

Props aaronrobertshaw, audrasjb, dmsnell, glendaviesnz, jonsurrell, ozgursar, ramonopoly, shailu25, westonruter.

Follow-up to [64544].

Fixes #64771.



git-svn-id: https://develop.svn.wordpress.org/trunk@62257 602fd350-edb4-49c9-b593-d223f7449a82