Remove title attributes and other accessibility improvements for the Authentication Check dialog "Close X"

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See related #24766 and all the following tickets about title attributes.

The "Close X" in the authentication check modal dialog has a title attribute that says "Close". It should be replaced with some hidden text, as already done in similar cases in the admin.

Additionally, it's just a <div> element and should be a button to natively support keyboard interaction and for better semantics.

See screenshot:

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The proposed patch removes the title attribute and uses a <button> instead of a <div>. Also simplifies the CSS and takes care of the hover/focus style.

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Screenshot before and after the patch:

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In 36014:

Accessibility: Improvements for the Authentication Check modal dialog "Close X".

Removes the title attribute and uses a <button> element instead of a <div>.
Also simplifies the CSS and takes care of the hover/focus style.

Fixes #35142.