autopilot: Use peer cache to perform autopilot Serf member work.

[jrasell]

Nomad 1.11 introduced a new peer cache to store information about Serf peers in order to perform server version checking. This can be used to perform the autopilot functionality which reduces the need to parse Serf tags and iterate members.

Autopilot needs the existing Serf members tags, so these have been added to the peer parts object. While this takes up a little space, it reduces our time complexity. The autopilot code has also been simplified, as the Raft version identifier is already in the server parts.

Links

Builds upon #26985

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