Fix getTurnServer response: return an integer ttl (#10922)

`ttl` must be an integer according to the OpenAPI spec:
https://github.com/matrix-org/matrix-doc/blob/old_master/data/api/client-server/voip.yaml#L70



True division (`/`) returns a float instead (`"ttl": 7200.0`).
Floor division (`//`) returns an integer, so the response is spec compliant.

Signed-off-by: Lukas Lihotzki <lukas@lihotzki.de>

changelog.d/10922.bugfix

+Fix a minor bug in the response to `/_matrix/client/r0/voip/turnServer`. Contributed by @lukaslihotzki.

synapse/rest/client/voip.py

@@ -70,7 +70,7 @@ class VoipRestServlet(RestServlet):
             {
                 "username": username,
                 "password": password,
-                "ttl": userLifetime / 1000,
+                "ttl": userLifetime // 1000,
                 "uris": turnUris,
             },
         )