Don’t worry, I know the answer…
There is a defect in the 2021 installer, where instead of checking to see if a later 2021.x is installed, it mistakingly checks for anything that is later than 2021.0. So, 2022.0 is seen by the 2021 installer as being a later version of 2021, and not an entire new version.
The work around is to uninstall 2022, uninstall 2021, reinstall 2022. Your workspace and plugins will still be ok.
We’re not going to go back and release an updated 2021 installer to solve the problem, so I think the work around is the only option.