chore: Use correct heading tag in Top navigation menu dropdown

[Al-Dani]

Description

Correct heading level provided for the menu dropdown in TopNavigation

Follow-up for #539

Related links, issue #, if available:AWSUI-19250

How has this been tested?

Manually

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

Base: 92.98% // Head: 92.98% // No change to project coverage 👍

Coverage data is based on head (26abe8b) compared to base (5fced58).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #641   +/-   ##
=======================================
  Coverage   92.98%   92.98%           
=======================================
  Files         574      574           
  Lines       16753    16753           
  Branches     4620     4620           
=======================================
  Hits        15577    15577           
  Misses       1099     1099           
  Partials       77       77           

Impacted Files	Coverage Δ
src/button-dropdown/internal.tsx	100.00% <ø> (ø)

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