doc: add release note about removal of `deprecatedrpc=walletwarningfield` flag

Co-authored-by: Jon Atack <jon@atack.com>

doc/release-notes-27757.md

@@ -0,0 +1,8 @@
+Wallet
+------
+
+- The `deprecatedrpc=walletwarningfield` configuration option has been removed.
+  The `createwallet`, `loadwallet`, `restorewallet` and `unloadwallet` RPCs no
+  longer return the "warning" string field. The same information is provided
+  through the "warnings" field added in v25.0, which returns a JSON array of
+  strings. The "warning" string field was deprecated also in v25.0. (#27757)