ci: Use Go 1.20 in GitHub Actions workflows

[halvards]

This also required upgrading GolangCI-Lint, which in turn found an unused field that appears to have previously gone undetected.

Related: #8558

[codecov]

Codecov Report

Merging #8691 (88523e3) into main (290280e) will decrease coverage by 6.01%.
The diff coverage is 54.27%.

@@            Coverage Diff             @@
##             main    #8691      +/-   ##
==========================================
- Coverage   70.48%   64.48%   -6.01%     
==========================================
  Files         515      616     +101     
  Lines       23150    30985    +7835     
==========================================
+ Hits        16317    19980    +3663     
- Misses       5776     9505    +3729     
- Partials     1057     1500     +443     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/completion.go	13.04% <0.00%> (-1.25%)	⬇️
cmd/skaffold/app/cmd/config/list.go	65.21% <ø> (ø)
cmd/skaffold/app/cmd/config/set.go	88.72% <ø> (ø)
cmd/skaffold/app/cmd/config/util.go	54.28% <ø> (ø)
cmd/skaffold/app/cmd/credits.go	100.00% <ø> (ø)
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/generate_pipeline.go	60.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_modules.go	65.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_profiles.go	66.66% <ø> (ø)
... and 39 more

... and 407 files with indirect coverage changes

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