docs: Update actions/checkout references to v5

[LeonStoldt]

I noticed that actions/checkout is still referenced as v4 and submit this PR to update to v5 according to https://github.com/actions/checkout/releases/tag/v5.0.0

Also there are references in github actions test.yml and publish.yml. If not managed by an automatic dependency update system like dependabot, I could also add an update for the references in the pipeline.