doc: add release notes for version restrictions

Author: kwvg

doc/release-notes-6729.md

@@ -0,0 +1,12 @@
+Notable Changes
+---------------
+
+* Dash Core will no longer permit the registration of new legacy scheme masternodes after the deployment of the v23
+  fork. Existing basic scheme masternodes will also be prohibited from downgrading to the legacy scheme after the
+  deployment is active.
+
+Updated RPCs
+----------------
+
+* `protx revoke` will now use the legacy scheme version for legacy masternodes instead of the defaulting to the
+   highest `ProUpRevTx` version.