Merge #17578: rpc: simplify getaddressinfo labels, deprecate previous behavior

8925df86c4df16b1070343fef8e4d238f3cc3bd1 doc: update release notes (Jon Atack)
8bb405bbadf11391ccba7b334b4cfe66dc85b390 test: getaddressinfo labels purpose deprecation test (Jon Atack)
60aba1f2f11529add115d963d05599130288ae28 rpc: simplify getaddressinfo labels, deprecate previous behavior (Jon Atack)
7851f14ccf2bcd1e9b2ad48e5e08881be06d9d21 rpc: incorporate review feedback from PR 17283 (Jon Atack)

Pull request description:

  This PR builds on #17283 (now merged) and is followed by #17585.

  It modifies the value returned by rpc getaddressinfo `labels` to an array of label name strings and deprecates the previous behavior of returning an array of JSON hash structures containing label `name` and address `purpose` key/value pairs.

  before
  ```
    "labels": [
      {
        "name": "DOUBLE SPEND",
        "purpose": "receive"
      }
  ```
  after
  ```
    "labels": [
      "DOUBLE SPEND"
    ]
  ```

  The deprecated behavior can be re-enabled by starting bitcoind with `-deprecatedrpc=labelspurpose`.

  For context, see:
  - https://github.com/bitcoin/bitcoin/pull/17283#issuecomment-554458001
  - http://www.erisian.com.au/bitcoin-core-dev/log-2019-12-13.html#l-425 (lines 425-427)
  - http://www.erisian.com.au/bitcoin-core-dev/log-2019-11-22.html#l-622

  Reviewers: This PR may be tested manually by building, then running bitcoind with and without the `-deprecatedrpc=labelspurpose` flag while verifying the rpc getaddressinfo help text and `labels` output.

  Next steps: deprecate the rpc getaddressinfo `label` field (EDIT: done in #17585) and add support for multiple labels per address. This PR will unblock those.

ACKs for top commit:
  jnewbery:
    reACK 8925df8
  promag:
    Code review ACK 8925df86c4df16b1070343fef8e4d238f3cc3bd1.
  meshcollider:
    Code review ACK 8925df86c4df16b1070343fef8e4d238f3cc3bd1

Tree-SHA512: c2b717209996da32b6484de7bb8800e7048410f9ce6afdb3e02a6866bd4a8f2c730f905fca27b10b877b91cf407f546e69e8c4feb9cd934325a6c71c166bd438

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