Mark Default Admin beta and revert the revert

Problem/Motivation

Default Admin should be marked beta. Most of the work remaining is reducing technical debt. Releasing the theme as experimental earlier would allow us to get more early testing on it before Drupal 12 where we are planning to remove Claro.

Proposed resolution

Per @catch, steps are:

1. It should be git show hash > remove.patch
2. Manually delete lines from the patch for anything that's staying removed.
3. patch -p1 -R < remove.patch
4. git add -p
5. Give or take phpstan and things.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet