Mention removed community/group worker endpoints in upgrade.md (matrix-org#13023)

Author: olmari

changelog.d/13023.doc

@@ -0,0 +1 @@
+Mention removed community/group worker endpoints in upgrade.md. Contributed by @olmari.

docs/upgrade.md

@@ -89,6 +89,24 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+# Upgrading to v1.61.0
+
+## Removal of depracated community/groups
+
+This release of Synapse will remove deprecated community/groups from codebase.
+
+### Worker endpoints
+
+For those who have deployed workers, following worker endpoints will no longer be existing
+and they are expected to be removed from reverse proxy config:
+
+-   `^/_matrix/federation/v1/get_groups_publicised$`
+-   `^/_matrix/client/(r0|v3|unstable)/joined_groups$`
+-   `^/_matrix/client/(r0|v3|unstable)/publicised_groups$`
+-   `^/_matrix/client/(r0|v3|unstable)/publicised_groups/`
+-   `^/_matrix/federation/v1/groups/`
+-   `^/_matrix/client/(r0|v3|unstable)/groups/`
+
 # Upgrading to v1.60.0
 
 ## Adding a new unique index to `state_group_edges` could fail if your database is corrupted