chore: use github-action for golangci-lint workflow

[odubajDT]

This PR

• move golangci-lint check to a separate workflow
• set ubuntu version in workflows to stable one
• fixes some of the linter issues
• ignores some of the linter issues

Notes

The reason why some of the linter issues are ignored is the awaited huge refactoring/changes in the code planned as part of this and this ticket. Most of the functions/files affected by lint ignorers will be changed and adapted, so there is no reason to fix them now. The fixing of the linter issues (for now ignored by //nolint comments) are part of the mentioned issues (see AC)

[codecov]

Codecov Report

Merging #538 (b0971c5) into main (3acd492) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #538   +/-   ##
=======================================
  Coverage   82.81%   82.81%           
=======================================
  Files          24       24           
  Lines        1437     1437           
=======================================
  Hits         1190     1190           
  Misses        209      209           
  Partials       38       38           

Files	Coverage Δ
...pis/core/v1alpha1/flagsourceconfiguration_types.go	97.09% <ø> (ø)
controllers/common/common.go	100.00% <ø> (ø)
controllers/common/flagd-injector.go	91.04% <ø> (ø)
...ollers/core/featureflagconfiguration/controller.go	54.63% <ø> (ø)
...rollers/core/flagsourceconfiguration/controller.go	46.37% <ø> (ø)
webhooks/featureflagconfiguration_webhook.go	68.75% <ø> (ø)
webhooks/pod_webhook.go	89.63% <ø> (ø)
webhooks/pod_webhook_deprecated.go	54.76% <ø> (ø)

Flag	Coverage Δ
unit-tests	82.81% <ø> (ø)

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