optimistically aim this change at v1.60.0

Signed-off-by: jesopo <github@lolnerd.net>

Author: jesopo

docs/modules/spam_checker_callbacks.md

@@ -251,7 +251,7 @@ any of the subsequent implementations of this callback.
 
 ### `drop_federated_event`
 
-_First introduced in Synapse v1.?.?_
+_First introduced in Synapse v1.60.0_
 
 ```python
 async def drop_federated_event(event: "synapse.events.EventBase") -> bool