doc: update release notes

Update the release notes regarding the change in rpc getaddressinfo `labels`.

Author: jonatack

doc/release-notes-17578.md

@@ -0,0 +1,8 @@
+Deprecated or removed RPCs
+--------------------------
+
+- The `getaddressinfo` RPC `labels` field now returns an array of label name
+  strings. Previously, it returned an array of JSON objects containing `name` and
+  `purpose` key/value pairs, which is now deprecated and will be removed in
+  0.21. To re-enable the previous behavior, launch bitcoind with
+  `-deprecatedrpc=labelspurpose`.