Reason
Per T345274: Remove similar-users service from k8s, this service was decommissioned from Wikimedia production and is unowned and unused. Let's archive it.

Note that the Similarusers project is also used for the replacement work for it, https://gerrit.wikimedia.org/g/mediawiki/extensions/SimilarEditors, which is not archived.

To-do list

• This task
  • Add maintainers of that service as subscribers to this task (if they have no apparent Phabricator account, notify them on-wiki or via email if possible and note it here).
  • Add Phabricator project <#similarusers> as a tag for this task
• On-wiki documentation
  • Archive https://wikitech.wikimedia.org/wiki/Similarusers
• Phabricator
  • Mark all relevant Phabricator tasks for the service Declined.
• Configuration/tests/integrations/etc.
  • Remove tests in integration/config (zuul/layout.yaml).
• Repositories
  • Empty repository, leaving only the README indicating the repo is archived, and referring to this task by URL.
  • Mark the repository read-only in Gerrit (also edit description to prepend "[ARCHIVED]" and a reference to this task, remove all access lists and reparent on All-Archived-Project): https://gerrit.wikimedia.org/r/admin/repos/mediawiki/services/similar-users,general
  • Deactivate the Diffusion repository for the service: https://phabricator.wikimedia.org/diffusion/MSSU/
  • Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-services-similar-users