Fix deprecation warnings on PHP 8.1

[arjanvlek]

If running on PHP 8.1, we would get deprecation messages like

PHP Deprecated:  Return type of SPSS\Sav\Record\Info::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in {...}/vendor/tiamo/spss/src/Sav/Record/Info.php on line 55

By adding a #[\ReturnTypeWillChange] attribute, we remain compatible with PHP 5.6+ but don't get deprecation warnings on 8.1.