refactor: Use native return types instead of using #[ReturnTypeWillChange]

[paulbalandan]

Description
The ReturnTypeWillChange attribute introduced in PHP 8.1 allows silencing of deprecation errors on PHP internal methods that will be using tentative return types to allow time to migrate code. I think this is now the time to do the migration.

Also, in case of union return types but the actual return is narrower, I used the narrower type.

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