Remove the Olivero theme

Problem/Motivation

Remaining tasks

1. The change record for this issue should include a link to recommendations page, https://www.drupal.org/node/3223395#s-olivero. (For example, the CR for removing HAL)
2. Tag this issue 'Needs release note.'
3. Remove the extension ;-).
4. Update update path tests as needed. See Learn how to write an automated update test.
5. Remove references from core/phpstan-baseline.neon.
6. Remove any spelling words specific to the extension from the dictionaries.
7. Add the extension to the relevant removed list, either DRUPAL_CORE_REMOVED_MODULE_LIST or DRUPAL_CORE_REMOVED_THEME_LIST, in system.install.
8. Check for references in @todo.

Release notes snippet

The EXTENSION_NAME {module/theme}has been removed from core, and can now be installed as a contrib {module/theme}.