Bump actions/checkout from 2 to 4 (#58)

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

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
     timeout-minutes: 30
     steps:
     - name: "Git Checkout"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
     - name: "Install Flutter"
       run: |
         sh .github/workflows/scripts/install-flutter.sh stable

.github/workflows/firestore_tests.yml

@@ -16,7 +16,7 @@ jobs:
     timeout-minutes: 5
     steps:
     - name: "checkout"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
     - name: "install flutter"
       uses: subosito/flutter-action@v2
     - name: "Run widget tests"