synapse complement image: hardcode enabling msc3266 (#17105)

This is an alternative to
https://github.com/matrix-org/matrix-rust-sdk/issues/3340 where we don't
need to change our CI setup too much in the Rust SDK repository, and
still can test MSC3266.

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

@@ -102,6 +102,8 @@ experimental_features:
   msc3874_enabled: true
   # no UIA for x-signing upload for the first time
   msc3967_enabled: true
+  # Expose a room summary for public rooms
+  msc3266_enabled: true
 
   msc4115_membership_on_events: true