Use $http3 variable instead of $quic

See https://quic.nginx.org/README:

    In http, an additional variable is available: $http3.
    The value of $http3 is "h3" for HTTP/3 connections,
    "hq" for hq connections, or an empty string otherwise.

Prevent the following from being reported:

nginx: [emerg] unknown "quic" variable

tests/https.conf

@@ -21,7 +21,7 @@ server {
 
     # Add Alt-Svc header to negotiate HTTP/3.
     add_header alt-svc 'h3-27=":8889"; ma=86400, h3-28=":8889"; ma=86400, h3-29=":8889"; ma=86400';
-    add_header QUIC-Status $quic;     # Sent when QUIC was used
+    add_header QUIC-Status $http3;     # Sent when QUIC was used
 
     location / {
         root   /static;