Bump actions/checkout from 3 to 4 (#388)

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>

.github/workflows/dart.yml

@@ -15,7 +15,7 @@ jobs:
         # Test with at least the declared minimum Dart version
         sdk: [2.18.7, stable]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: dart-lang/setup-dart@v1
         with:
           sdk: ${{ matrix.sdk }}

.github/workflows/publish_demos.yml

@@ -10,7 +10,7 @@ jobs:
       image: dart
     steps:
       - name: Checkout 🛎️
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install rsync 📚
         run: |

.github/workflows/release_unreleased_prs.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 2
           ref: master

.github/workflows/triage.yml

@@ -8,7 +8,7 @@ jobs:
     name: Assign Rob
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Apply untriaged label
         uses: actions/github-script@v6
         with: