Remove Bartik from Drupal core

Problem/Motivation

See 🌱 [Meta] Tasks to remove Bartik from core and move to contrib Fixed and #3249109: Deprecate Bartik → . Now that Olivero is the default theme for Drupal core, it's time to move Bartik to a contributed project.

Steps to reproduce

Proposed resolution

Remaining tasks

3. Remove the module ;-).
4. Remove references from core/phpstan-baseline.neon.

User interface changes

API changes

Data model changes

Release notes snippet

The Bartik theme has been removed from Drupal 10.0.0 🌱 [Meta] Tasks to remove Bartik from core and move to contrib Fixed , and is now available as a contributed theme. Sites using Bartik should install the contributed theme prior to updating to Drupal 10.