[ceph-users] Release notes update: don't (try to) upgrade from v19.2.6 to Tentacle < 20.2.4

Hi, I know it might be unlikely, but not all users are (yet) subscribed to this list or for some reason can't catch up. I feel like the docs should clearly state that you can't upgrade from 19.2.6 to Tentacle < 20.2.4. I'm almost sure that there will be users tying to do exactly that just because 20 > 19. :-) Regards, Eugen