[meta] Tasks to deprecate Claro

Problem/Motivation

The actual deprecation of Claro is postponed on #3576488: [meta] Admin theme: path to beta and stable, however it may be possible to work on some steps prior to that issue being fixed.

Track the steps needed to deprecate extension Claro. See Remove a core extension and move it to a contributed project of the deprecation policy.

The removal of extension Claro was approved in #3576460: [policy, no patch] Deprecate and remove Claro.

All the child issues here, except in the infrastructure project, are "11.5.0 release priority".

Remaining tasks

To see all the files using Claro.

git grep -lwi claro | grep -v core/themes/claro/ | grep -v core/assets | grep -v phpstan-baseline | grep -v MAINTAINERS

1. ✅ Find someone to maintain the contrib version of the extension. smustgrave has agreed to do this #11
2. ✅ Add the extension to the list of the Upcoming Deprecations.
3. Move integrations implemented by other modules to the extension.
   Create child issues or child meta issues, as needed, to address the following points. Not all points will apply to all extensions.
   • #3582093: Convert Unit and Kernel tests that use Claro to Admin
   • #3582098: Update core/tests that use Claro to Admin
   • #3582153: Update Functional tests in modules that use Claro to Admin
   • #3621198: Change remaining usages of Claro
   • #3622220: Use Admin theme in installer related tests
   • Use in the Admin theme are being addressed in the meta to get that stable. #3576496: Remove references to Claro
   • Move tests to the extension.
   • Help and help topics. #3582158: Change use of Claro in help text
   • #3618971: Use Admin theme in installer
   • #3621981: Remove admin-custom.css
   • ImplementationNameTest.php - See #3623097: Remove remaining Gin and Claro implementation names from Default Admin theme, part 2
   • Database update page #3622756: Use Admin theme for the update page
   • For a theme, move theme-specific integration to the theme.
   • Remove the extension from one or more profiles. #3582101: Make Admin the default theme in profiles and recipes
   • Remove references to the extension from database dumps. #3624072: Remove claro from database dumps
   • Remove templates from the extension’s markup.
     • Remove templates from themes that are staying in core, leave them in deprecated themes
     • Keep skipping the template in the stable copies test.
4. ✅Do a thorough search of core for any remaining references to the extension. If references are found, outside of the extension, then create issues to remove the references.
5. ✅ Create the contrib project with a stable release, before the alpha version of the major release. Follow the process in Create the contrib project with a stable release for creating the sub tree split.
6. ✅ Deprecate the core extension. #3576670: Deprecate Claro theme
7. ✅Create an issue, 'Ensure that Claro does not get special core treatment' in the packages.drupal.org issue queue to ensure that the composer namespace for the contributed project matches the core one after removal. This is to allocate the drupal/claro Composer namespace to contrib extension rather than the core extension.
8. ✅Update the Deprecated and obsolete extensions