Only use block markup for comment form button when using a block theme

[noisysocks]

Description

Follows #37245.

Fixes a small bug I noticed thanks to a failing test in WordPress/wordpress-develop#2048.

Steps to reproduce the bug:

1. Activate an older theme e.g. Twenty Ten.
2. Go to a post on the frontend.
3. Look at the "Post Comment" button.

It looks weird because we're erroneously changing the markup to be a Gutenberg button. We should only do that if the current theme is a block theme.

How has this been tested?

As above.

Screenshots

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).