Metadata API: Bump deprecation version to next likely candidate

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tuf/api/metadata.py

@@ -395,7 +395,7 @@ def verify_delegate(
         """Verify that ``delegated_metadata`` is signed with the required
         threshold of keys for ``delegated_role``.
 
-        .. deprecated:: 2.2.0
+        .. deprecated:: 3.1.0
            Please use ``Root.verify_delegate()`` or ``Targets.verify_delegate()``.
         """