Keep GHC release candidates in supported versions

haskell-ci supports GHC release candidates so I sometimes include the latest RC in my project CI. The RC is installed via ghcup. Last time, when 9.2.1 came out, the release candidate 9.2.0.xyz disappeared quickly from ghcup. This is a bit unfortunate, because then CI runs will suddenly fail. Ideally, ghcup would be monotonic, so only new versions are added, never any removed (like hackage). I think the RCs do not have to be supported forever, but it would be good if they were supported for sufficient time, say, a year.