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

8925df86c4 doc: update release notes (Jon Atack)
8bb405bbad test: getaddressinfo labels purpose deprecation test (Jon Atack)
60aba1f2f1 rpc: simplify getaddressinfo labels, deprecate previous behavior (Jon Atack)
7851f14ccf 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 8925df86c4.
  meshcollider:
    Code review ACK 8925df86c4

Tree-SHA512: c2b717209996da32b6484de7bb8800e7048410f9ce6afdb3e02a6866bd4a8f2c730f905fca27b10b877b91cf407f546e69e8c4feb9cd934325a6c71c166bd438

test/functional/wallet_labels.py

@@ -13,10 +13,8 @@ from collections import defaultdict
 
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_raises_rpc_error
-from test_framework.wallet_util import (
-    labels_value,
-    test_address,
-)
+from test_framework.wallet_util import test_address
+
 
 class WalletLabelsTest(BitcoinTestFramework):
     def set_test_params(self):
@@ -157,12 +155,7 @@ class Label:
         if self.receive_address is not None:
             assert self.receive_address in self.addresses
         for address in self.addresses:
-            test_address(
-                node,
-                address,
-                label=self.name,
-                labels=labels_value(name=self.name, purpose=self.purpose[address])
-             )
+            test_address(node, address, label=self.name, labels=[self.name])
         assert self.name in node.listlabels()
         assert_equal(
             node.getaddressesbylabel(self.name),