Upgrade actions/checkout

Author: byjg

.github/workflows/phpunit.yml

@@ -28,7 +28,7 @@ jobs:
 #          - "5.6"
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: composer install
       - run: ./vendor/bin/phpunit
 
@@ -39,5 +39,5 @@ jobs:
     env:
       DOC_GITHUB_TOKEN: '${{ secrets.DOC_TOKEN }}'
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: curl https://opensource.byjg.com/add-doc.sh | bash /dev/stdin php singleton