Theme JSON: Update core theme json resolver class use to Gutenberg version

[aaronrobertshaw]

What?

Updates some calls to the core WP_Theme_JSON_Resolver class with its core equivalent: WP_Theme_JSON_Resolver_Gutenberg

Why?

• Makes sure that the code in Gutenberg is using the Gutenberg resolver class
• Consistency

How?

Append _Gutenberg to the core class name.

Testing Instructions

1. Turn on the new post dataviews page experiment
2. Navigate to Gutenberg > Posts
3. Ensure that Global Styles are correctly displayed in the post preview
4. Double check all e2es are passing

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Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: mikachan <mikachan@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

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[mikachan]

The code changes look good to me and everything is testing well. I tested with the dataviews experiment turned on, and Global Styles were correctly displayed in the previews ✅

I also ran the e2e tests locally, and they were all passing and I can see they're passing on this PR (including the font-library.spec.js test!) ✅

[andrewserong]

Testing nicely for me, too!

[aaronrobertshaw]

Thanks for the reviews and getting this one merged while I was away. Appreciate it 🙇