Update to checkout v3

Author: gem-uhe

.github/workflows/FHIRValidator.yml

@@ -25,7 +25,7 @@ jobs:
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       # Java and .NET are already installed on ubuntu-latest