[Meta] Tasks to remove Bartik from core and move to contrib

Problem/Motivation

This issue is to coordinate and track the steps needed to remove Bartik from core to contrib. The steps to remove a theme and move it to contrib are in development, the steps for modules is being used as a guide. See Remove a core module and move it to a contributed project → of the deprecation policy.

The removal of Bartik was approved in TBA

Proposed resolution

Completed tasks

User interface changes

API changes

Data model changes

Release notes snippet