Remove shared rooms info from upgrade/workers doc as it's still experimental (#8290)

UPGRADE.rst

-Upgrading to v1.20.0
-====================
-
-Shared rooms endpoint (MSC2666)
--------------------------------
-
-This release contains a new unstable endpoint `/_matrix/client/unstable/uk.half-shot.msc2666/user/shared_rooms/.*`
-for fetching rooms one user has in common with another. This feature requires the
-`update_user_directory` config flag to be `True`. If you are you are using a `synapse.app.user_dir`
-worker, requests to this endpoint must be handled by that worker. 
-See `docs/workers.md <docs/workers.md>`_ for more details.
-
-
 Upgrading Synapse
 =================
 

docs/workers.md

@@ -380,7 +380,6 @@ Handles searches in the user directory. It can handle REST endpoints matching
 the following regular expressions:
 
     ^/_matrix/client/(api/v1|r0|unstable)/user_directory/search$
-    ^/_matrix/client/unstable/uk.half-shot.msc2666/user/shared_rooms/.*$
 
 When using this worker you must also set `update_user_directory: False` in the
 shared configuration file to stop the main synapse running background