Update actions/checkout action to v4 (#8)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/main.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: subosito/flutter-action@v2
         with:
           channel: 'stable'
@@ -23,7 +23,7 @@ jobs:
     runs-on: windows-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: subosito/flutter-action@v2
         with:
           channel: 'stable'
@@ -37,7 +37,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: subosito/flutter-action@v2
         with:
           channel: 'stable'