Bundled Themes: Update Twenty Twenty.

* Add `.has-background` excluding condition to wide and full group-blocks.

This brings Twenty Twenty in sync with GitHub. For a full list of changes since [46714], see https://github.com/WordPress/twentytwenty/compare/162edc0...fa8c3fc.

Props allancole, karmatosed.
See #48110, #48386, #48450, #48505.
Fixes #48557.
Built from https://develop.svn.wordpress.org/trunk@46715


git-svn-id: https://core.svn.wordpress.org/trunk@46515 1a063a9b-81f0-0310-95a4-ce76da25c4cd