Add missing `x-addedInMatrixVersion` to `servers_in_room` in `v2/send_join` (#1398)

* Add missing `x-addedInMatrixVersion` to `servers_in_room` in `v2/send_join`

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

* Add changelog

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

changelogs/server_server/newsfragments/1398.feature

@@ -0,0 +1 @@
+Extend `/_matrix/federation/v2/send_join` to allow omitting membership events, per [MSC3706](https://github.com/matrix-org/matrix-doc/pull/3706).

data/api/server-server/joins-v2.yaml

@@ -255,6 +255,7 @@ paths:
                   server's signature.
               servers_in_room:
                 type: array
+                x-addedInMatrixVersion: "1.6"
                 description: |-
                   **Required** if `members_omitted` is true.