go.{mod,sum}, .github: bump minimum golang version to 1.20.

[klihub]

• bump minimum golang version to 1.20
• test using 1.20, 1.21, and 1.22 in CI
• bump setup-go to v5
• update golangci-lint and fix related linter errors

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.98%. Comparing base (53d3371) to head (6d9fd80). Report is 8 commits behind head on main.

:exclamation: Current head 6d9fd80 differs from pull request most recent head 8a9d653

Please upload reports for the commit 8a9d653 to get more accurate results.

Files	Patch %	Lines
pkg/adaptation/result.go	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #88 +/- ## ========================================== - Coverage 64.55% 63.98% -0.57% ========================================== Files 10 4 -6 Lines 1845 1305 -540 ========================================== - Hits 1191 835 -356 + Misses 503 356 -147 + Partials 151 114 -37 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.