[REST API compatibility] special handling for _freeze

[joegallo]

Follow up to #78918, replaces #74705

The squash merge commit in this middle of this PR is just #79859, you can ignore it here and review it there.

#78918 removed the _freeze endpoint entirely, which means we respond to _freeze requests with a 400 Bad Request and a response body like {"error":"no handler found for uri [/tweets-1/_freeze] and method [POST]"} -- that is, the standard Elasticsearch "you've got the wrong method and/or url" response.

That's definitely the right treatment for a normal request, but if the client specifies compatible-with=7 on their request, then we should have a better response -- I've gone with 410 Gone and:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "unsupported_operation_exception",
        "reason" : "It is no longer possible to freeze indices, but existing frozen indices can still be unfrozen"
      }
    ],
    "type" : "unsupported_operation_exception",
    "reason" : "It is no longer possible to freeze indices, but existing frozen indices can still be unfrozen"
  },
  "status" : 410
}

[elasticmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[jakelandis]

LGTM