chore: bump actions/checkout from 2 to 4

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] <support@github.com>

.github/workflows/codecov.yml

@@ -6,7 +6,7 @@ jobs:
   upload:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Flutter
         uses: subosito/flutter-action@v2
         with:

.github/workflows/gh-pages.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2 # Only works with v2
+      - uses: actions/checkout@v4 # Only works with v2
       - uses: subosito/flutter-action@v2
       - uses: bluefireteam/flutter-gh-pages@v8
         with:

.github/workflows/publish.yml

@@ -11,7 +11,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Publish
         uses: k-paxian/dart-package-publisher@master
         with:

.github/workflows/verification.yml

@@ -6,7 +6,7 @@ jobs:
   verify:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Flutter
         uses: subosito/flutter-action@v2
         with: