Update actions/checkout to v3

Author: TiagoCavalcante

.github/workflows/backstopjs.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Use Node.js 16
         uses: actions/setup-node@v2