fix: Remove deprecation annotation on side navigation externalIconAriaLabel link property

[SpyZzey]

Description

In #3975, externalIconAriaLabel in the side navigations "link" has been annotated with @deprecated. However, we do not support an alternative for it as #3975 only added the i18n support on an application level, not a per-component basis.

Related links, issue #, if available: AWSUI-61823

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (f7c23e9) to head (89f0361).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4392   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files         909      909           
  Lines       26637    26637           
  Branches     9615     9615           
=======================================
  Hits        25958    25958           
  Misses        636      636           
  Partials       43       43           

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