fix: explicitly enable httpmuxgo121

[jkroepke]

What this PR does / why we need it

ref: #680 (comment)

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

• fixes #

Special notes for your reviewer

Particularly user-facing changes

Checklist

Complete these before marking the PR as ready to review:

• DCO signed
• The PR title has a summary of the changes
• The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.54%. Comparing base (e47b030) to head (ef3bd8f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #706      +/-   ##
==========================================
- Coverage   84.60%   84.54%   -0.06%     
==========================================
  Files          58       58              
  Lines        3442     3442              
==========================================
- Hits         2912     2910       -2     
- Misses        370      371       +1     
- Partials      160      161       +1     

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