Upgrade to actions/checkout@v4

Author: davidhalter

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           submodules: recursive
 
@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           submodules: recursive