.github: Bump actions/checkout to v3

Author: bd808

.github/workflows/main.yml

@@ -32,7 +32,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y libyaml-dev
       - name: Checkout repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Setup PHP
         uses: "shivammathur/setup-php@v2"
         with: