Specify the room_version response property on /make_{leave|join}

Original proposal: https://github.com/matrix-org/matrix-doc/pull/1813
Implementation proof: https://github.com/matrix-org/synapse/pull/4447

There are no changes from the original proposal.

api/server-server/joins.yaml

@@ -65,6 +65,12 @@ paths:
           schema:
             type: object
             properties:
+              room_version:
+                type: string
+                description: |-
+                  The version of the room where the server is trying to join. If not provided,
+                  the room version is assumed to be either "1" or "2".
+                example: "2"
               event:
                 allOf:
                   - $ref: "definitions/unsigned_pdu.yaml"
@@ -143,7 +149,8 @@ paths:
                       - state_key
           examples:
             application/json: {
-              event: {
+              "room_version": "2",
+              "event": {
                 "$ref": "examples/unsigned_pdu.json",
                 "type": "m.room.member",
                 "state_key": "@someone:example.org",

api/server-server/leaving.yaml

@@ -57,6 +57,12 @@ paths:
           schema:
             type: object
             properties:
+              room_version:
+                type: string
+                description: |-
+                  The version of the room where the server is trying to leave. If not provided,
+                  the room version is assumed to be either "1" or "2".
+                example: "2"
               event:
                 allOf:
                   - $ref: "definitions/unsigned_pdu.yaml"
@@ -131,6 +137,7 @@ paths:
                       - state_key
           examples:
             application/json: {
+              "room_version": "2",
               "event": {
                 "$ref": "examples/unsigned_pdu.json",
                 "type": "m.room.member",