feat: By default, new targets now emit a warning message if they do n…

…ot support `ACTIVATE_VERSION`

singer_sdk/_singerlib/encoding/_base.py

@@ -117,12 +117,19 @@ def _process_record_message(self, message_dict: dict) -> None: ...
     @abc.abstractmethod
     def _process_state_message(self, message_dict: dict) -> None: ...
 
-    @abc.abstractmethod
-    def _process_activate_version_message(self, message_dict: dict) -> None: ...
-
     @abc.abstractmethod
     def _process_batch_message(self, message_dict: dict) -> None: ...
 
+    def _process_activate_version_message(self, message_dict: dict) -> None:  # noqa: ARG002, PLR6301
+        """Default implementation for ACTIVATE_VERSION messages.
+
+        Override this method in a subclass to handle ACTIVATE_VERSION messages.
+
+        Args:
+            message_dict: Dictionary representation of the Singer message.
+        """
+        logger.warning("ACTIVATE_VERSION messages are not supported")
+
     def _process_unknown_message(self, message_dict: dict) -> None:  # noqa: PLR6301
         """Internal method to process unknown message types from a Singer tap.