ci: Upgrade checkout action to v3

[bmuschko]

What this PR does / why we need it:

Upgrades checkout action to v3

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #2605

Special notes for your reviewer:

None

[bmuschko]

Looks like the check "Semantic PR" is failing. What exactly is expected here?

[ritazh]

Looks like the check "Semantic PR" is failing. What exactly is expected here?

The title needs to be prefixed with values from https://github.com/open-policy-agent/gatekeeper/blob/master/.github/semantic.yml e.g. chore or ci

[ritazh]

LGTM

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.06 ⚠️

Comparison is base (e33c551) 53.36% compared to head (24c4f87) 53.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2658      +/-   ##
==========================================
- Coverage   53.36%   53.30%   -0.06%     
==========================================
  Files         120      120              
  Lines       10594    10594              
==========================================
- Hits         5653     5647       -6     
- Misses       4508     4513       +5     
- Partials      433      434       +1     

Flag	Coverage Δ
unittests	53.30% <ø> (-0.06%)	⬇️

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see 2 files with indirect coverage changes

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[sozercan]

Thank you!

[bmuschko]

@ritazh Thanks, I'll keep that in mind for my next contribution. I think it would make sense to mention the commit message expectations explicitly in the contribution guide here.

[ritazh]

@ritazh Thanks, I'll keep that in mind for my next contribution. I think it would make sense to mention the commit message expectations explicitly in the contribution guide here.

Feel free to update the docs or ask questions if we can make contribution easier.
Re: Semantic PR, it's documented here https://open-policy-agent.github.io/gatekeeper/website/docs/help/#contributing-process

Create a GitHub Pull Request for your change, following the instructions in the pull request template and use semantic PR title