JsonBomb: Don't directly type hint `mixed'

Under PHP 7.x this causes an error:

    Return value of JsonBomb::jsonSerialize() must be an instance of
    mixed, resource returned

Because `mixed' is not recognized as an actual type hint before PHP 8.

src/Json/JsonBomb.php

@@ -15,7 +15,10 @@
  */
 final class JsonBomb implements JsonSerializable
 {
-    public function jsonSerialize(): mixed
+    /**
+     * @return mixed
+     */
+    public function jsonSerialize()
     {
         return fopen('php://memory', 'w');
     }