Only enable MSC1767 room version if the config flag is on

Using a similar technique to knocking:
https://github.com/matrix-org/synapse/pull/6739/files#diff-3af529eedb0e00279bafb7369370c9654b37792af8eafa0925400e9281d57f0a

Author: turt2live

synapse/config/experimental.py

@@ -16,6 +16,7 @@
 
 import attr
 
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersions
 from synapse.config._base import Config
 from synapse.types import JsonDict
 
@@ -131,3 +132,7 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
 
         # MSC1767 and friends: Extensible Events
         self.msc1767_enabled: bool = experimental.get("msc1767_enabled", False)
+        if self.msc1767_enabled:
+            # Enable room version (and thus applicable push rules from MSC3931/3932)
+            version_id = RoomVersions.MSC1767v10.identifier
+            KNOWN_ROOM_VERSIONS[version_id] = RoomVersions.MSC1767v10