tests(grpc): fix failing test by replacing listen ... http2 by `htt…

…p2 on` to avoid deprecation message

To avoid message:

[warn] the "listen ... http2" directive is deprecated, use the "http2" directive instead in /home/runner/work/lua-kong-nginx-module/lua-kong-nginx-module/t/servroot/conf/nginx.conf:28

t/003-grpc.t

@@ -231,7 +231,8 @@ falseincorrect argument, expects a string, got number
     lua_package_path "../lua-resty-core/lib/?.lua;lualib/?.lua;;";
 
     server {
-        listen 127.0.0.1:12346 http2;
+        listen 127.0.0.1:12346;
+        http2 on;
         server_name   example.com;
 
         server_tokens off;