fix: Center side navigation header text vertically when icon is present

[jperals]

Description

Because icon and header text in Side navigation are both anchored to the top, if the icon is taller the header looks like this:

After this PR, it looks like this:

When the icon is not taller than the header text, there should be no visual changes.

Ticket: AWSUI-20266

How has this been tested?

• Added new permutations
• Verified Backstop failures, they are expected because of the new permutations. Our pipeline visual regression tests should not fail because they compare each permutation individually.

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

Patch and project coverage have no change.

Comparison is base (fe5918e) 93.97% compared to head (cd5b416) 93.97%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1412   +/-   ##
=======================================
  Coverage   93.97%   93.97%           
=======================================
  Files         642      642           
  Lines       17313    17313           
  Branches     5691     5691           
=======================================
  Hits        16270    16270           
  Misses        973      973           
  Partials       70       70           

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