fix: Use semantic heading for side navigation header

[avinashbot]

Description

We hadn't done this in the past because having an <h2> persistently before the <h1> in the DOM order would lead to a confusing page hierarchy. But all signals now point to DOM order not being as important as the meaningful hierarchy of the title itself, so... let's do it

Related links, issue #, if available: AWSUI-19376

How has this been tested?

Only a small change in the HTML tag used; can't think of a test case that wouldn't be a brittle one. Also, there should be no visual changes caused by this change either.

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: 92.97% // Head: 92.98% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (1f0c4c7) compared to base (d52870f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #661   +/-   ##
=======================================
  Coverage   92.97%   92.98%           
=======================================
  Files         574      574           
  Lines       16766    16781   +15     
  Branches     4628     4631    +3     
=======================================
+ Hits        15588    15603   +15     
  Misses       1101     1101           
  Partials       77       77           

Impacted Files	Coverage Δ
src/side-navigation/internal.tsx	100.00% <ø> (ø)
src/input/index.tsx	100.00% <0.00%> (ø)
src/input/internal.tsx	100.00% <0.00%> (ø)
src/modal/internal.tsx	100.00% <0.00%> (ø)
src/container/index.tsx	100.00% <0.00%> (ø)
src/tag-editor/index.tsx	90.81% <0.00%> (ø)
src/autosuggest/internal.tsx	95.23% <0.00%> (ø)
src/text-filter/internal.tsx	100.00% <0.00%> (ø)
src/property-filter/token-editor.tsx	98.13% <0.00%> (ø)
src/internal/components/tab-trap/index.tsx	100.00% <0.00%> (ø)
... and 7 more

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