Follow-up from "Proto: update delegate deactivation table with the actual last activity cycle"

The following discussion from !18946 (merged) should be addressed:

• @dianegalloiswong started a discussion:

  Storage.Contract.Delegate_last_cycle_before_deactivation's name is no longer appropriate. I think at least the module should be renamed to something like for instance Delegate_last_recorded_activity to help with future protocol modifications. iirc the storage table can be renamed too without affecting its content but I'm not sure; if not, it's not a big deal to keep the old name.

  Can be done in a separate MR to avoid conflicts with the one already on top of this one.