chore: update actions/checkout to v6 across all workflows

Several workflows still use actions/checkout v4.1.7 (or v4.0.0). Update them all to v6.0.2.

Workflows to update:
- `apidocs-upload.yml`
- `apidocs.yml`
- `codeql.yml`
- `docker.yml`
- `integration-test.yml`
- `lint.yml`
- `main.yml`