chore: Remove last remnants of IE11 support

[TrevorBurnham]

Support for IE11 was dropped years ago. This commit removes lingering legacy code aimed at that ancient browser.

How has this been tested?

✅ All unit tests pass.

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 97.10%. Comparing base (4ff58c7) to head (73e15a9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3791   +/-   ##
=======================================
  Coverage   97.10%   97.10%           
=======================================
  Files         839      839           
  Lines       24418    24418           
  Branches     8623     8624    +1     
=======================================
  Hits        23710    23710           
+ Misses        701      658   -43     
- Partials        7       50   +43     

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