Lower the priority of the gallery gap css

[glendaviesnz]

Fixes the order that the Gallery gap CSS is enqueued so it is not overwritten by layout CSS in non block themes.

Merged into Gutenberg trunk and adding here for backporting to WP 6.0.1

Testing

• In a non-block theme, add a custom CSS var or --wp--style--gallery-gap-default: 37px;
• Add a Gallery and check in the frontend that the gap is set to 37px
• Also try and block theme and make sure gap still works as expected

[adamziel]

@glendaviesnz Thank you for preparing this PR! I only realized now that the updated PHP file is in wp-includes/blocks directory which means it's automatically copied from the relevant Gutenberg directory on build. This means we're good and this PR can be closed. Sorry for the trouble!