Bundled Themes: Add theme support for responsive embeds

Add `add_theme_support( 'responsive-embeds' );` to all of the responsive default themes as part of their block editor support. This makes sure any embeds coming from the new block-based editor maintain their aspect ratios at different screen sizes.

Merges [43868] into trunk.

Props laurelfulford.
Fixes #45274.
Built from https://develop.svn.wordpress.org/trunk@44227


git-svn-id: https://core.svn.wordpress.org/trunk@44057 1a063a9b-81f0-0310-95a4-ce76da25c4cd