gateway: upgrade time lib

[Terkwood]

Deal with deprecation warning

warning: use of deprecated item 'time::precise_time_ns': Use `OffsetDateTime::now() - OffsetDateTime::unix_epoch()` to get a `Duration` since a known epoch.
   --> src/websocket.rs:569:27
    |
569 |                 let now = time::precise_time_ns();
    |                           ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default