Mark links that point to the current URL in bundled theme headers with `aria-current="page"` to inform screen reader users that this link is the current page.
Changes to Twenty Ten through Twenty Twenty add `aria-current` and `rel="home"` where appropriate, Twenty Twenty One only adds `rel="home"`.
Props bschneidewind, joedolson, hiabhaykulkarni, sabernhardt.
Fixes#62895.
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