The `aria-current` attribute is a simple, effective way to communicate to assistive
technologies which the current item within a set of items is. While the admin menu
structure isn't 100% ideal (the top-level item is repeated also as a sub-item)
adding `aria-current="page"` to the sub-items gives an important feedback to
assistive technologies users.
This change introduces `aria-current` for the first time in core. Worth noting
there are other places where it can be used to improve accessibility. These should
be addressed in separate tickets and patches.
Fixes#41589.
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