Deprecate Bartik

Problem/Motivation

See the parent issue for all the tasks that are needed to deprecate Bartik.

Steps to reproduce

Proposed resolution

Remaining tasks

Work on the patch is postponed on #3278124: Convert various tests that use bartik/seven to olivero/claro but there is other work that can be done.

1. Create a section on Deprecated and obsolete themes and themes to provide recommendations for sites using theme Bartik. The recommendation are to include instructions for sites using Bartik and for contributed themes that depend on Bartik.
2. Set lifecycle to deprecated.
3. Set lifecylcle_link to the section added above, https://www.drupal.org/node//3223395#s-Bartik.
4. The change record for this issue includes a link to the doc page.

User interface changes

API changes

Data model changes

Release notes snippet