fix: PHP 8.1 deprecation warning

[bednar]

Closes #108

Proposed Changes

Fixed deprecation warning ArrayAccess.

The public function offsetGet($offset) cannot be changed to public offsetGet(mixed $offset): mixed because the mixed is supported as type from PHP 8.0.

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] make test completes successfully
• [x] Commit messages are in semantic format
• [x] Sign CLA (if not already signed)