nginx: explicitly use stable version

Also updates the documention of the NixOS option `services.nginx.package`
that upstream recommends using the mainline version instead.

Fixes #21665.

pkgs/servers/http/nginx/generic.nix

@@ -62,6 +62,6 @@ stdenv.mkDerivation {
     homepage    = http://nginx.org;
     license     = licenses.bsd2;
     platforms   = platforms.all;
-    maintainers = with maintainers; [ thoughtpolice raskin ];
+    maintainers = with maintainers; [ thoughtpolice raskin fpletz ];
   };
 }