Update actions/checkout action to v3 (#200)

Co-authored-by: Renovate Bot <[email protected]>
Co-authored-by: Daichi Furiya <[email protected]>

Author: 3 people

.github/workflows/dart-ci.yaml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         os: [ ubuntu-latest, windows-latest, macos-latest ]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: subosito/flutter-action@v2
         with:
           flutter-version: '2.8.1'

.github/workflows/release.yaml

@@ -22,7 +22,7 @@ jobs:
             target: x86_64-unknown-linux-gnu
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: subosito/flutter-action@v2
         with:
           flutter-version: '2.8.1'