This provides more flexibility when writing tests and benefits from running in a front-end context which is necessary for some tests like checking that updates are supported. Additionally, this provides a more robust interface for developers who want to integrate with Site Health tests.
Because the `wp/v2` endpoint is reserved for modeling core entities, site health is registered in its own `wp-site-health/v1` namespace.
The existing ajax actions have been maintained for backward compatibility.
Props Clorith, chrisvanpatten, afragen, pokhriyal, TimothyBlynJacobs.
Fixes#48105.
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