Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/greetings.yml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       - name: Check out repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Greet first-time contributors
         id: greet