Adopt raft-autopilot library

[notnoop]

Nomad currently directly imports Consul's private autopilot package (github.com/hashicorp/consul/agent/consul/autopilot). Last month, Consul extracted autopilot to be its own module to ease adoption in other products: https://github.com/hashicorp/raft-autopilot

Let's move off our direct dependency on the internal repo in preference to the new module.

[tgross]

Note to implementor: probably worth doing #13219 at the same time.

[shoenig]

Took a brief look into making the switch; looks like it's going to be a bit more complicated than just renaming imports. In addition to raft-autopilot there is also raft-autopilot-enterprise, and the features referenced by Nomad OSS in the old Consul autopilot library have been split up between the two.

Still very much want to make this happen - without it Nomad is forced to import consul v1.7 😞

[github-actions]

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