When Internet Explorer encounters a non interactive element with a `tabindex`
attribute, it adds the element to the accessibility tree with a `role=group` and
an accessible name computed from the element. This prevents JAWS from announcing
any fieldset legend within the element.
- removes `tabindex="0"` from the content and the toolbar containers: these tabindex attributes are no longer needed
- removes `aria-label="Main content"` from the content container: not needed
- keeps the media modal focus fallback introduced in [38142] by making the `#wpbody-content` element focusable only when needed
Props stevefaulkner, aardrian.
Fixes#43154.
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