update github workflow action

- update to v4 of checkout action to avoid node version warnings
- perform apt-get update to ensure fresh package list and locations

.github/workflows/validate-xml.yml

@@ -12,10 +12,12 @@ jobs:
 
     steps:
     - name: Checkout Repository
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Install xmllint
-      run: sudo apt-get install -y libxml2-utils
+      run: |
+        sudo apt-get update 
+        sudo apt-get install -y libxml2-utils
 
     - name: Validate XML Files
       run: |