Bump actions/checkout from 2 to 3 (#202)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2...v3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/downstream.yml

@@ -18,7 +18,7 @@ jobs:
     steps:
     # Step 1: run a standard checkout action, provided by github
     - name: Checkout main
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
       with:
         ref: main
         # submodules: 'recursive'     ### may be needed in your situation