Support PHP 8.1 better

[xvilo]

Currently this library emits a lot of warnings on PHP. An example:

Deprecated: Return type of FluentDOM\Serializer\Factory\Group::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /src/fluentdom/fluentdom/src/FluentDOM/Serializer/Factory/Group.php on line 75

These should be fixed

[beerendlauwers]

@ThomasWeinert Do you happen to have time for this? This interferes a lot with our automated code checking.

[Rikardsen]

I would also like FluentDOM to work with PHP 8

[ThomasWeinert]

Please check the refactor/php8 branch.