Fix deprecation notice required after optional parameters

[D-Groenewegen]

Issue

This (minor) issue is about https://github.com/SemanticMediaWiki/SemanticFormsSelect/blob/master/src/SemanticFormsSelectInput.php#L72

In PHP 8 adding required after optional parameters in a function/method has become deprecated (1). I don't know what's preferable as a solution: reshuffle the sequence or convert the required parameters to optional ones by adding defaults. I'll leave that up to your good judgment.