nixos/dokuwiki: use php 8.4

PHP 8.1 needs to be removed since it'll be EOLed by the end of the year.

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

@@ -56,7 +56,7 @@ let
     pkgs.writeTextFile {
       inherit name;
       text = "<?php\n${text}";
-      checkPhase = "${pkgs.php81}/bin/php --syntax-check $target";
+      checkPhase = "${pkgs.php84}/bin/php --syntax-check $target";
     };
 
   mkPhpValue =
@@ -368,8 +368,7 @@ let
         };
 
         phpPackage = mkPackageOption pkgs "php" {
-          default = "php81";
-          example = "php82";
+          default = "php84";
         };
 
         phpOptions = mkOption {