chore: Enable 'modernize'; bump golangci-lint to v2.6.1

[alexandear]

This PR bumps golangci-lint to v2.6.1 and fixes modernize lint issues.

The omitzero check is not easy to fix, so I disabled it with the TODO comment.

[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (5953276) to head (f508f0d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
example/commitpr/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3817   +/-   ##
=======================================
  Coverage   92.27%   92.27%           
=======================================
  Files         192      192           
  Lines       13896    13896           
=======================================
  Hits        12823    12823           
  Misses        884      884           
  Partials      189      189           

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