Add PHP return type declarations, and enforce them in lint

## Feature Description

The plugin runs on PHP 7.4 and up, so a method can declare the type it returns. The codebase doesn't use return type declarations.

A package like `slevomat/coding-standard` ships a fixable sniff that writes the type from the `@return` docblock. `rector/rector` can read the method body instead of the docblock, which should be safer.

Make a return type the rule here, so every method states what it returns. Add a lint check that reports a method without one, and update the code already written.

This is a follow-up to [a review comment on #13118](https://github.com/google/site-kit-wp/pull/13118#discussion_r3614153271), which asked for this ticket.

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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Start by reviewing the PHP codebase's existing lint configuration and the available Rector and slevomat/coding-standard approaches. Determine how to add a check for missing return types and update existing methods across the codebase; done means every method declares a return type and lint reports omissions.

Written by the indexing model from the issue text.