Fix requestOpenIdToken response: integer expires_in (#10175)

`expires_in` must be an integer according to the OpenAPI spec:
https://github.com/matrix-org/matrix-doc/blob/master/data/api/client-server/definitions/openid_token.yaml#L32

True division (`/`) returns a float instead (`"expires_in": 3600.0`).
Floor division (`//`) returns an integer, so the response is spec compliant.

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

changelog.d/10175.bugfix

@@ -0,0 +1 @@
+Fix a minor bug in the response to `/_matrix/client/r0/user/{user}/openid/request_token`. Contributed by @lukaslihotzki.

synapse/rest/client/v2_alpha/openid.py

@@ -85,7 +85,7 @@ class IdTokenServlet(RestServlet):
                 "access_token": token,
                 "token_type": "Bearer",
                 "matrix_server_name": self.server_name,
-                "expires_in": self.EXPIRES_MS / 1000,
+                "expires_in": self.EXPIRES_MS // 1000,
             },
         )