Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/CI.yml

@@ -16,7 +16,7 @@ jobs:
         node: ['12', '14', '16']
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Cache .pnpm-store
         uses: actions/cache@v2