Fix: Suppress PHP8.1+ notice for jsonSerialize

[flack]

Currently, on PHP8.1 and later, you get the following deprecation notice:

( ! ) Deprecated: Return type of StellarWP\Validation\ValidationRuleSet::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in src/ValidationRuleSet.php on line 264

We can't add mixed return type to the function, because that would not work in PHP7, so we just add a suppression as recommended by the error message

[JasonTheAdams]

This is great, @flack! Thank you for submitting this as well as providing a clear explanation!