Today when upgrading to the next major version we have a so-called
_major version barrier_: once the cluster comprises nodes of the new
major version then nodes of the previous major version are prevented
from joining the cluster. This means we can be certain that
`clusterState.nodes().getMinNodeVersion().major` will never decrease, so
we can implement upgrade logic that relies on the cluster remaining in
its wholly-upgraded state.
This commit generalises this behaviour to apply to all upgrades, so that
we can be certain that `clusterState.nodes().getMinNodeVersion()` will
never decrease in a running cluster.
Closes#72911
e027ce977c