Bump actions/checkout from 2 to 4 (#306)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v4)

---
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/ci.yml

@@ -27,7 +27,7 @@ jobs:
             version: '1'
             arch: x86
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v1
         with:
           version: ${{ matrix.version }}
@@ -52,7 +52,7 @@ jobs:
   #   name: Documentation
   #   runs-on: ubuntu-latest
   #   steps:
-  #     - uses: actions/checkout@v2
+  #     - uses: actions/checkout@v4
   #     - uses: julia-actions/setup-julia@v1
   #       with:
   #         version: '1'