Add MSC3820 (room version 11) option 2 unstable room version. (#15666)

changelog.d/15666.misc

@@ -0,0 +1 @@
+Implement "option 2" for [MSC3820](https://github.com/matrix-org/matrix-spec-proposals/pull/3820): Room version 11.

synapse/api/room_versions.py

@@ -485,6 +485,30 @@ class RoomVersions:
         msc3931_push_features=(),
         msc3989_redaction_rules=True,
     )
+    MSC3820opt2 = RoomVersion(
+        # Based upon v10
+        "org.matrix.msc3820.opt2",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.ROOM_V4_PLUS,
+        StateResolutionVersions.V2,
+        enforce_key_validity=True,
+        special_case_aliases_auth=False,
+        strict_canonicaljson=True,
+        limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=True,  # Used by MSC3820
+        msc2176_redaction_rules=True,  # Used by MSC3820
+        msc3083_join_rules=True,
+        msc3375_redaction_rules=True,
+        msc2403_knocking=True,
+        msc2716_historical=False,
+        msc2716_redactions=False,
+        msc3389_relation_redactions=False,
+        msc3787_knock_restricted_join_rule=True,
+        msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=True,  # Used by MSC3820
+        msc3931_push_features=(),
+        msc3989_redaction_rules=True,  # Used by MSC3820
+    )
 
 
 KNOWN_ROOM_VERSIONS: Dict[str, RoomVersion] = {