[taskchampion] Drop old versions on the sync server

(Summary updated Jan 2024)

The sync model results in a linked sequence of versions on the server (whether taskchampion-sync-server or on a cloud service). To avoid that history growing forever, there's some cleanup (in `taskchampion/taskchampion/src/server/cloud/server.rs` for cloud services) that applies a date-based heuristic to delete old versions. That heuristic is trading use of extra space against the possibility that a replica that hasn't synced in a loooong time will find itself unable to sync, because the last version it saw has been deleted.

The proposal here involves tracking all replicas and the last version they saw. The downside to this proposal is that it adds some required UI for users to manage replicas.

See https://github.com/taskchampion/taskchampion/issues/311 for more background.