The WebSocket masks need to be generated by random_bytes

[robrwo]

The RFC for websockets states that the random numbers should be unpredictable. The only vulnerability is cache-poisoning in a non-encrypted environment.

However, since 9.46 Mojo::Util has a rand0m_bytes function that returns bytes from the system CSPRNG. This change should have been included in #2317 when that was added.

This is a small and simple change that ticks a box for RFC compliance.

See #2326.

[kraih]

There are no known exploits, and the performance impact of the change is unclear.