Fix PHPStan level 0 issues for Customizer setting subclasses by returning correct types

[westonruter]

The changes in this PR were cherry-picked (e.g. 2f72407 and 22370b6) from #10419 for Core-61175.

This Customizer ensures return type consistency for the update methods for setting classes. In practice, these methods are protected and the return value is never read, so it's just about housekeeping. It keeps the overridden methods compatible with the parent class's method.

Trac ticket: https://core.trac.wordpress.org/ticket/64238

Use of AI Tools

n/a

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[github-actions]

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Props westonruter.

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[github-actions]

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[westonruter]

@justlevine Would you give this a review for me?

[peterwilsoncc]

This looks good to me and is testing well:

Testing notes:

• Activate 2014
• Add header image
• Confirm
• Remove header image
• Confirm
• Add a menu
• Place it in header (primary)
• confirm
• Add items to menu
• confirm

Confirm indicates:

• dynamically updated in the customizer
• saved when publish button hit
• displayed on front end after hitting publsih

[westonruter]

Committed in r61670 (ff7564a)