GHA: Bump actions/checkout from 2 to 4 in the gha-dependencies group

Bumps the gha-dependencies group with 1 update: [actions/checkout](https://github.com/actions/checkout).


Updates `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
  dependency-group: gha-dependencies
...

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

.github/workflows/dart_ci.yaml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         sdk: [2.19.6, stable]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: dart-lang/setup-dart@v1
         with:
           sdk: ${{ matrix.sdk }}