PHP 8.1 compatibility

Author: alexhernandezord

src/Serializer/Date.php

@@ -42,7 +42,7 @@ public function asDateTime()
         return $dateTime;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize():mixed
     {
         return $this->value;
     }

src/Serializer/Decimal.php

@@ -33,7 +33,7 @@ public function asFloat()
         return $this->value / 100;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize():mixed
     {
         return $this->value;
     }