Allow joining node to trigger term bump

[DaveCTurner]

In rare circumstances it is possible for an isolated node to have a greater
term than the currently-elected leader. Today such a node will attempt to join
the cluster but will not offer a vote to the leader and will reject its cluster
state publications due to their stale term. This situation persists since there
is no mechanism for the joining node to inform the leader that its term is
stale and a new election is required.

This commit adds the current term of the joining node to the join request. Once
the join has been validated, the leader will perform another election to
increase its term far enough to allow the isolated node to join properly.

Fixes #53271

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Cluster Coordination)

[DaveCTurner]

Note to reviewers: I contemplated a few alternative mechanisms for telling the master that it needs to bump its term in these circumstances. I chose this one because it's a small and reasonably clear change, but it does have the disadvantage that the join attempt that triggers the term bump will fail and require a retry. IMO this is rare enough that that's ok. I also considered

• a more specialised exception from the publication indicating that the term bump is needed.
• piggybacking the term on the fault detection mechanism instead of the join.
• a whole new transport message specifically for this situation, sent prior to/instead of the join.

Liveness is hard.

[ywelsch]

LGTM