Bump actions/checkout from 3 to 4 (#81)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

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

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

Author: dependabot[bot]

.github/workflows/build.yaml

@@ -18,7 +18,7 @@ jobs:
       - uses: dart-lang/[email protected]
         with:
           sdk: ${{ matrix.sdk }}
-      - uses: actions/checkout@v4.1.1
+      - uses: actions/checkout@v4
       - run: dart --version
       - run: dart pub get
       # Verify the library re-generates.

.github/workflows/jekyll-gh-pages.yml

@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup Pages
         uses: actions/configure-pages@v3
       - name: Build with Jekyll