Mention callbacks introduced in v1.42.0

According to #10524

Author: babolivier

docs/modules/presence_router_callbacks.md

@@ -10,6 +10,8 @@ The available presence router callbacks are:
 
 ### `get_users_for_states`
 
+_First introduced in Synapse v1.42.0_
+
 ```python 
 async def get_users_for_states(
     state_updates: Iterable["synapse.api.UserPresenceState"],
@@ -30,6 +32,8 @@ Synapse concatenates the sets associated with this key from each dictionary.
 
 ### `get_interested_users`
 
+_First introduced in Synapse v1.42.0_
+
 ```python
 async def get_interested_users(
     user_id: str