workflow: Use actions/checkout@v4 (#9)

Author: ryonakano

.github/workflows/update.yml

@@ -20,10 +20,10 @@ jobs:
         apt-get install -y git devscripts debootstrap germinate
 
     - name: Checkout platform
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Checkout metapackages
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
       with:
         repository: elementary/metapackages
         path: metapackages