Users sometimes attempt to downgrade a node in place, but downgrades are
totally untested and unsupported and generally don't work. We protect
against this by recording the node version in the metadata and refusing
to start if we encounter metadata written by a future version.

However in 8.0 (elastic#42489) we changed the directory layout so that a 7.x
node won't find the upgraded metadata, or indeed any other data, and
will proceed as if it's a fresh node. That's almost certainly not what
the user wants, so with this commit we create a file at
`${path.data}/nodes` at each startup, preventing an older node from
starting.

Closes elastic#52414