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>