Backport of make version checks specific to region (1.4.x) into release/1.4.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #14912 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

The below text is copied from the body of the original PR.

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• One-time tokens are not replicated between regions, so we don't want to enforce
  that the version check across all of serf, just members in the same region.
• Scheduler: Disconnected clients handling is specific to a single region, so we
  don't want to enforce that the version check across all of serf, just members in
  the same region.
• Variables: enforce version check in Apply RPC
• Cleans up a bunch of legacy checks.

This changeset is specific to 1.4.x and the changes for previous versions of
Nomad will be manually backported in separate PRs.

[github-actions]

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