Unsupported upgrades from 7.nonlast to 8.x are still permitted #81865
Description
Upgrading to the next major version with a rolling restart requires upgrading to the last minor of the previous major first. However you could upgrade from 5.x to 6.x and from 6.x to 7.x via a full cluster restart without upgrading to 5.last (respectively 6.last) first.
For 8.x we will impose the same restriction on both upgrade paths: they must always go via 7.last. This will ensure that folks are notified about all deprecations and other changes before starting the major upgrade.
We should prevent users from inadvertently doing an unsupported upgrade from 7.15 or earlier to 8.x by bailing out if the on-disk node version is too old. It should be ok to let users explicitly bypass this restriction by running elasticsearch-node override-version, but we want this to be an explicit action with a disclaimer and an acknowledgement from the user, rather than something that just silently appears to work.
/cc @skearns64
Relates #81862