Encapsulate all audience-related reset logic

## Feature Description

In #8180, we implemented a reset mechanism for all data related to Audience Segmentation when deactivating the Analytics module or changing its property. To do this, a `Modules\Analytics_4\Reset_Audiences` class was introduced. While it was originally specced to reset module-level AS settings within this class as well, it was ultimately moved to the `Modules\Analytics_4::register` method as explained [here](https://github.com/google/site-kit-wp/pull/9328#discussion_r1767528626).

This issue is a follow-up for #8180, where we move the reset of module-level AS settings to `Modules\Analytics_4\Reset_Audiences` as originally specced, updating the `Modules\Analytics_4\Settings::merge` (or even `Core\Modules\Module_Settings::merge` if deemed unproblematic) in the process.

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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 with Modules\Analytics_4\Reset_Audiences and the module-level reset currently handled in Modules\Analytics_4::register. Read the discussion in #8180 and pull request #9328, then inspect Analytics_4\Settings::merge and Core\Modules\Module_Settings::merge before choosing the scope. Done means all audience-related reset logic is handled by Reset_Audiences without changing the existing deactivation or property-change behavior.

Written by the indexing model from the issue text.