Update actions to use major version checkout@v4

Change-Id: If68972ac325ae45a7de269d80d4305d9e80ff0af
diff --git a/.github/workflows/ci_test.yml b/.github/workflows/ci_test.yml
index 68be103..43237b4 100644
--- a/.github/workflows/ci_test.yml
+++ b/.github/workflows/ci_test.yml
@@ -7,7 +7,8 @@
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Krill
-        uses: actions/checkout@v2
+        # Reference the major version of a release
+        uses: actions/checkout@v4
 
       - name: Set up JDK 17
         uses: actions/setup-java@v1