Reason: Per T180573: Convert ActivityMonitor to use extension registration.
- Archive documentation on mediawiki.org - https://www.mediawiki.org/wiki/Extension:ActivityMonitor
- Remove from mediawiki/extensions (along with in .gitmodules)
- Remove tests in integration/config (zuul/layout.yaml)
- Remove from translatewiki/groups/MediaWiki/mediawiki-extensions.txt
- Empty repository and add a OBSOLETE file.
- Archive Phabricator project MediaWiki-extensions-ActivityMonitor for associated extension/skin
- Edit Phabricator project MediaWiki-extensions-ActivityMonitor description for associated extension with a link to this ticket
- Archive the extension's diffusion repository rEACM extension-ActivityMonitor
- Mark the repository read-only in Gerrit.
- Delete the GitHub mirror.