Add a v6 event format with correct depth limits (#2114)

* Add a v6 event format with correct `depth` limits

* Changelog

* numbers are hard

data/api/server-server/definitions/pdu_v3.yaml

@@ -24,6 +24,13 @@ allOf:
         type: string
         description: For redaction events, the ID of the event being redacted.
         example: "$def/456+oldevent"
+      depth:
+        type: integer
+        description: |-
+          The maximum depth of the `prev_events`, plus one. Must be less than the
+          maximum value for an integer (2^63 - 1). If the room's depth is already at
+          the limit, the depth must be set to the limit.
+        example: 12
       auth_events:
         type: array
         items: