Fix M_WRONG_ROOM_KEYS_VERSION error not including current_version field

changelog.d/11988.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where `M_WRONG_ROOM_KEYS_VERSION` errors would not include the specced `current_version` field.

synapse/api/errors.py

@@ -406,6 +406,9 @@ def __init__(self, current_version: str):
         super().__init__(403, "Wrong room_keys version", Codes.WRONG_ROOM_KEYS_VERSION)
         self.current_version = current_version
 
+    def error_dict(self) -> "JsonDict":
+        return cs_error(self.msg, self.errcode, current_version=self.current_version)
+
 
 class UnsupportedRoomVersionError(SynapseError):
     """The client's request to create a room used a room version that the server does