update: Upgrade Cargo dependencies to latest compatible versions (#176)

Runs 'cargo update' and bumps direct-dep floors in Cargo.toml:
- tokio 1.50.0 -> 1.51.1
- signal-hook 0.4.3 -> 0.4.4
- fastrand 2.3.0 -> 2.4.1

All other transitive updates picked up via Cargo.lock refresh
(socket2 0.6.1 -> 0.6.3, bytes 1.11.0 -> 1.11.1, etc).

rand 0.8 -> 0.10 is a breaking API change (rand::thread_rng was
removed); deferred to a dedicated migration.

Author: inureyes