feat(api): add Delete and Sub methods, update CI workflows

- Add Delete(key) to remove keys from all config layers
- Add Sub(key) to get a scoped ConfigManager for nested sections
- Update CodeQL workflow actions to v4/v5/v3
- Update govulncheck workflow checkout to v4
- Remove boilerplate comments from CodeQL workflow
- Add comprehensive tests for both new methods
- Update README with Sub() usage example and API docs

.github/workflows/govulncheck.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     name: Run govulncheck
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - id: govulncheck
         uses: golang/govulncheck-action@v1
         with: