The Site Health score indicator isn't exactly a "progress bar" and shouldn't use ARIA roles and properties related to progress bars. Also, some browser / screen reader combinations don't announce the score properly.
- removes any ARIA
- adds a screen-reader-text "Current health score:"
- adds `role="img" aria-hidden="true" focusable="false"` to the SVG
- reduces CSS specificity simplifying unnecessary overqualified selectors
- fixes the syntax for ::after and ::before (double colon)
Fixes#46621.
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