ci: pin to npm@11.6.0

[pichlermarc]

Which problem is this PR solving?

Pins to npm@11.6.0 to avoid npm/cli#8628. When updating the lockfile to v3, I was met with quite a few missing "optional" dependencies for both @bufbuild/buf and nx that were previously installed.

I'm not sure if it's the same change in npm that caused it, but I've also seen an out-of-sync package-lock.json for older npm versions in #5976

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.17%. Comparing base (8110127) to head (b918c81).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5979   +/-   ##
=======================================
  Coverage   95.17%   95.17%           
=======================================
  Files         316      316           
  Lines        8521     8521           
  Branches     1763     1763           
=======================================
  Hits         8110     8110           
  Misses        411      411           

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