review gitolite retirement progress and send a reminder

As part of the Gitolite retirement procedure (TPA-RFC-36, #41180 (closed)), review the progress of the migration and send a reminder:

• how many repositories are left to migrate, populating #41215 (closed) with the result
• did any repository get changes since the deprecation notice on 2023-06-08
• send a reminder, similar to #41212 (closed)