[meta] Tasks to deprecate the Olivero theme

Problem/Motivation

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

The removal of extension Olivero was approved in #3590816: [policy, no patch] Deprecate Olivero and move to contrib.

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

Remaining tasks

To find uses of olivero:

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

1. Begin finding someone to maintain the contrib version of the extension.
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.
   • Move tests to the extension.
     • #3407067: message.js should work with whitespace in empty message wrappers
     • #3614949: Move tests to Olivero, part 1
     • #3614964: Remove install of Olivero from tests
   • Move Help Topics to the extension.
   • For a theme, move theme-specific integration to the theme.
   • Remove the extension from one or more profiles. #3590882: Standard profile should not include Olivero anymore, should provide guidance on how to get started
   • Remove references to the extension 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.#3595083: Deprecate the Olivero theme
7. Create an issue, 'Ensure that Olivero 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/olivero Composer namespace to contrib extension rather than the core extension.
8. Update the Deprecated and obsolete extensions