Drupal 12 Compatibility for Radix 5

Problem/Motivation

Drupal 12 is scheduled for release the week of December 7, 2026. Radix 5's
info.yml currently declares:

core_version_requirement: ^8.7.7 || ^9.0 || ^10 || ^11

Sites wanting to upgrade to Drupal 12 will not be able to use Radix 5 until
this is updated, so we should confirm compatibility and bump the version
requirement ahead of the D12 release.

Proposed resolution

- Test Radix 5 against Drupal 12 (once beta/dev snapshots are available)
- Update core_version_requirement in radix.info.yml to include ^12
- Check for any deprecated APIs or removed core functions used by the theme
- Update the starterkit / subtheme generation scripts if affected

Remaining tasks

- [ ] Test against Drupal 12.x-dev
- [ ] Fix any deprecation errors found
- [ ] Update core_version_requirement