Fix (accessibility): Remove redundant aria-label that fails accessibility conformance

[maggiewachs]

Fixes #12955

Summary

• The "Improve this map" link fails accessibility standards defined in WCAG SC 2.5.3: Label in Name (Level A) because the visible link text is not included in the aria-label attribute.
• This error is present in every default installation of Mapbox that includes the attribution block:
  
• Removing the aria-label (a single line in the source) means we don't need to create a hacky workaround to eliminate the accessible name mismatch, and would benefit all users of Mapbox, especially those who rely on assistive tech (voice control, screen readers, etc)
• This fix may also improve Mapbox's feedback analytics :)

Verified locally

• the aria-label is no longer present on the "Improve this map" link
• screen reader announces the visible text correctly when the link is focused (VoiceOver with Safari, MacOS 14.1)
• Chrome DevTools correctly identifies the link's accessible name as "Improve this map", and matches the visible link text

Launch Checklist

• Make sure the PR title is descriptive and preferably reflects the change from the user's perspective.
• Add additional detail and context in the PR description (with screenshots/videos if there are visual changes).
• Manually test the debug page.
• Write tests for all new functionality and make sure the CI checks pass.
• Document any changes to public APIs.
• Post benchmark scores if the change could affect performance.
• Tag @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes.
• Tag @mapbox/gl-native if this PR includes shader changes or needs a native port.

[CLAassistant]

All committers have signed the CLA.

[stepankuzmin]

Thanks for the contribution, @maggiewachs!