Configure Synapse to run MSC4306: Thread Subscriptions Complement tests. (#18819)

Pairs with: https://github.com/matrix-org/complement/pull/795

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

docker/complement/conf/workers-shared-extra.yaml.j2

@@ -133,6 +133,8 @@ experimental_features:
   msc3984_appservice_key_query: true
   # Invite filtering
   msc4155_enabled: true
+  # Thread Subscriptions
+  msc4306_enabled: true
 
 server_notices:
   system_mxid_localpart: _server