Bundled Themes: Replace “Gutenberg” in code comments and classes.

This update replaces the term “Gutenberg” in the themes code comments and classes with something more future-friendly.

Props laurelfulford.

Merges [44213] into trunk.

Fixes #45452.
Built from https://develop.svn.wordpress.org/trunk@44313


git-svn-id: https://core.svn.wordpress.org/trunk@44143 1a063a9b-81f0-0310-95a4-ce76da25c4cd