fix(AppHeaderBanner): add focus-visible styles for keyboard accessibility

[waterWang]

Summary

Closes #48959

Adds :focus-visible styles to the announcement banner link in AppHeaderBanner to ensure keyboard users can see where focus is positioned.

Changes

• Added :focus-visible outline styles to the linkStyleOverrides in AppHeaderBanner.tsx
• Applied the same styles in the dark mode variant
• The outline uses a white 2px ring with 2px offset for clear visibility against the blue gradient background

Before

Focusing the announcement banner link via keyboard produced zero visual change — the default browser outline was suppressed with no author-defined replacement.

After

A white outline ring appears around the link when focused via keyboard (:focus-visible), satisfying WCAG 2.4.7 Focus Visible and 1.4.11 Non-Text Contrast.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-48962--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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