DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
Addresses: Issue #13810 1. Adds a last_used_at attribute to the Key table/model 2. Update a key's last_used_at whenever it gets used 3. Display how long ago an ssh key was last used