Update golangci-lint and re-enable CI linting step

[brandond]

Proposed Changes

Apparently we have not been linting in CI since December 2023, as there are a large number of issues to fix. It was apparently intended for this to be a temporary measure to unblock the 1.29 release, but we never opened an issue to track re-enabling it, so it never got done.

• Update Kubernetes to v1.29.0+k3s1 #9052

Total issue count by rule, excluding the tests/ dir, and after excluding some rules that we are not ready to enforce:

     36 unnecessary-format
     29 redundant-build-tag
     27 use-errors-new
     21 use-any
     20 empty-lines
     19 comment-spacings
     10 unexported-naming
     10 indent-error-flow
      9 bare-return
      7 redefines-builtin-id
      6 unnecessary-stmt
      6 defer
      5 deep-exit
      4 unchecked-type-assertion
      4 if-return
      4 exported
      4 duplicated-imports
      4 dot-imports
      3 import-alias-naming
      3 bool-literal-in-expr
      2 unexported-return
      2 superfluous-else
      1 useless-break
      1 unhandled-error
      1 struct-tag
      1 identical-switch-branches
      1 get-return
      1 File is not properly formatted

Types of Changes

CI

Verification

Testing

Linked Issues

User-Facing Change

Further Comments

[codecov]

Codecov Report

❌ Patch coverage is 24.04580% with 199 lines in your changes missing coverage. Please review.
✅ Project coverage is 21.49%. Comparing base (c0b1314) to head (d2acbc3).
⚠️ Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/agent/flannel/flannel.go	0.00%	18 Missing ⚠️
pkg/cluster/bootstrap.go	0.00%	17 Missing ⚠️
pkg/util/lru.go	45.83%	13 Missing ⚠️
pkg/cli/server/server.go	0.00%	10 Missing ⚠️
pkg/util/net.go	0.00%	7 Missing ⚠️
pkg/util/patch.go	0.00%	7 Missing ⚠️
pkg/vpn/vpn.go	0.00%	7 Missing ⚠️
pkg/agent/flannel/setup.go	0.00%	6 Missing ⚠️
pkg/agent/netpol/netpol.go	0.00%	6 Missing ⚠️
pkg/cgroups/cgroups_linux.go	0.00%	6 Missing ⚠️
... and 43 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13343      +/-   ##
==========================================
+ Coverage   21.42%   21.49%   +0.07%     
==========================================
  Files         187      188       +1     
  Lines       15296    15291       -5     
==========================================
+ Hits         3277     3287      +10     
+ Misses      11573    11558      -15     
  Partials      446      446              

Flag	Coverage Δ
unittests	21.49% <24.04%> (+0.07%)	⬆️

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[brandond]

level=warning msg="[runner] Can't process results by diff processor: can't prepare diff by revgrep: could not read git repo: error executing \"git merge-base main HEAD\": exit status 128: fatal: Not a valid object name main\n"

I guess this doesn't work with a sparse checkout; I'll fix that. Seems like it falls back to just linting everything though.