Gerrit/Moving projects to non-Gerrit-repo

Background
This page discribes the process to move projects from a Gerrit repo to a non-Gerrit repo, like Github or Gitlab. For previous discussion and example see T221549.

Actions to move a project from Gerrit to a non-Gerrit repo

 * Phabricator
 * Archive Phabricator project <#ProjectX> for the extension.
 * Mark all Phabricator tasks for the extension either Declined or Invalid.
 * Edit Phabricator project <#ProjectX> description for the extension/skin with a link to this ticket.


 * Translatewiki.net/translations
 * Switch translatewiki.net from Gerrit to GitHub (groups/MediaWiki/mediawiki-extensions.txt) (add translatewiki.net project as a tag for this task).


 * Configuration/tests/integrations/etc.
 * Remove from mediawiki/extensions / mediawiki (along with in .gitmodules).
 * Remove tests in integration/config (zuul/layout.yaml; use archive-repo.py in the  repo).
 * Remove manifest/other references from mediawiki/vagrant if they exist.
 * Remove from https://doc.wikimedia.org/cover-extensions/ if present (requires Continuous-Integration-Infrastructure shell user to delete directly from the server).


 * Repositories
 * Empty repository leaving only the README indicating the repo has moved to the new repo and referring to this task by URL.
 * Mark the repository read-only in Gerrit (Edit description to prepend "[MOVED]" and append Task reference).
 * Configure repository for the extension/skin: (rESTA extension-SemanticTasks) to watch GitHub upstream instead.