Support for HTTP/2, TLSv1.3, and use Unix sockets instead of HTTP

[binwiederhier]

Resolves binwiederhier/ntfy#552

[otbutz]

Also worth a try:

events {
        worker_connections 40500; # ntfy: 2x of LimitNOFILE
        use epoll;
        multi_accept on;
}

Source: https://github.com/patrickdappollonio/nginx-vs-caddy-benchmark/blob/main/nginx.conf

[binwiederhier]

@otbutz I'm already regretting taking this ticket on. Because now I have to do a release and all that just to roll this out. Let's do baby steps. I have zero performance problems, so let's not prematurely optimize. :-)