Is your feature request related to a problem? Please describe.

In principle we ought to be able to remove old versions from the below file when we move on to the next version. Removing old versions forces removing code-paths which are gated on those versions. That's a good forcing function. This represents a reasonable chunk of work scattered over many teams. This issue tracks the actual removal of the versions.

https://github.com/cockroachdb/cockroach/blob/master/pkg/clusterversion/cockroach_versions.go#L321