Deprecate notice in PHP8.3 on the TreeNodeMethodsTrait method offsetGet()

[RobinBrackez]

There's a deprecate notice when you run PHP8.3 with this bundle:

Deprecated: Return type of offsetGet(mixed $offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice.

I guess there can be an easy fix for this in file TreeNodeMethodsTrait.

[RobinBrackez]

Update: made a PR for this: #753