`wp_delete_user()` and `wpmu_delete_user()` both require an `$id` parameter.
Previously, the functions did not verify that the value passed was, in fact,
a number. As such, passing an object or any other entity that would be cast
to int `1` would result in user 1 being deleted. We fix this by enforcing
the requirement that `$id` be numeric.
Props dipesh.kakadiya, utkarshpatel, juliobox.
Fixes#33800.
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