Bump actions/checkout from 2 to 3 (#28)

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](actions/checkout@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>

Author: dependabot[bot]

.github/workflows/test-package.yml

@@ -22,7 +22,7 @@ jobs:
       matrix:
         sdk: [dev]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: dart-lang/setup-dart@v1
         with:
           sdk: ${{ matrix.sdk }}
@@ -49,7 +49,7 @@ jobs:
         os: [ubuntu-latest]
         sdk: [2.12.0, dev]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: dart-lang/setup-dart@v1
         with:
           sdk: ${{ matrix.sdk }}