Replace Websocket library

[fortuna]

We are using https://pkg.go.dev/golang.org/x/net/websocket, which is poorly maintained.

Better options are:

• https://pkg.go.dev/github.com/gorilla/websocket
• https://pkg.go.dev/nhooyr.io/websocket

Ideally we would be able to support Websockets over h2 and even h3, for multiplexing and better performance.

However, it's not clear any implementation can support it yet, given limitations in the http2 support in Go:

• proposal: x/net/websocket: implement support for websockets over HTTP/2 golang/go#53209
• proposal: x/net/http2: support for WebSockets over HTTP/2 golang/go#49918

[fortuna]

Oh, neither of the libraries support h2 :-/

• WebSockets over HTTP/2 nhooyr/websocket#4
• Websockets over HTTP/2 gorilla/websocket#417

[fortuna]

We can consider adding h3 support with quic-go: https://quic-go.net/docs/webtransport/