Dont finally work with php <8.0

[sHelllWalker]

Hi, i have some trouble to use your library for php 7.2 project. In your composer.json php require >7.1 Problem is in using return type hinting for AttributeGroup::jsonSerialize. mixed is available since php 8

Although you can use the library, it is impossible to process the response, because the only way to validly access it is through json_encode

[nateobray]

That's a great point... I see you made a pull request. Let me test and merge it. Thank you!