nixos/grav: use PHP 8.3

The stable version of Grav is not compatible with later versions of
PHP.

nixos/modules/services/web-apps/grav.nix

@@ -69,7 +69,7 @@ in
       '';
     };
 
-    phpPackage = mkPackageOption pkgs "php" { };
+    phpPackage = mkPackageOption pkgs "php83" { };
 
     maxUploadSize = mkOption {
       type = types.str;