nginx: Switch from $host to $http_host for HTTP/3 compatibility.

nginx sets the value of the `$http_host` variable to the empty string
when using http/3, as there is technically no `Host:` header sent:
https://github.com/nginx-quic/nginx-quic/issues/3

Users with a browser that support http/3 will send their first request
to nginx with http/2, and get an expected HTTP 200 -- but any
subsequent requests will fail with am HTTP 400, since the browser will
have upgraded to http/3, which has an empty `Host` header, which Zulip
rejects.

Switch to the `$host` variable, which works for all HTTP versions.

Co-authored-by: Alex Vandiver <alexmv@zulip.com>

scripts/lib/sharding.py

@@ -44,7 +44,7 @@ def write_updated_configs() -> None:
             sharding_json_f.write("{}\n")
             return
 
-        nginx_sharding_conf_f.write("map $http_host $tornado_server {\n")
+        nginx_sharding_conf_f.write("map $host $tornado_server {\n")
         nginx_sharding_conf_f.write("    default http://tornado9800;\n")
         shard_map: Dict[str, Union[int, List[int]]] = {}
         shard_regexes: List[Tuple[str, Union[int, List[int]]]] = []