Include HTTP_HOST $host in fastcgi_params

When using HTTP/3, the Host header is not sent anymore by the client, it always and only uses the :authority pseudo-header. Thus, fastcgi never receives it, and some PHP applications rely on it because it's mandatory on HTTP/1.1.

I propose to use fastcgi_param HTTP_HOST $host because as mentioned here (https://trac.nginx.org/nginx/ticket/2468#comment:1) it will use the real computed Host value from nginx.