Add ReturnTypeWillChange attribute for offsetGet to suppress notice

[westonruter]

I noticed a deprecation warning after #1585:

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

This adds #[ReturnTypeWillChange] attribute as suggested.

This is also used in core, for example: https://github.com/WordPress/wordpress-develop/blob/de51a4412dc3fa4d2591c59492fa76227cdddd29/src/wp-includes/class-wp-block-list.php#L81-L92

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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