refactor: use array-key as the benevolent union of int|string in array shapes

[paulbalandan]

Description
Plain array<int|string, ...> is actually different from the nature of PHP array keys, that's why PHPStan uses array-key or (int|string) to mean these kind of unioned types: benevolent unions.

Ref: https://phpstan.org/config-reference#checkbenevolentuniontypes

This PR normalizes to use array-key instead of plain union.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide