chore(): bump vm_pool_image from 20.04 → 24.04

- the 20.04 image has been removed from Azure DevOps
- the 24.04 image has proven to work on several other Java projects

Author: RichardSlater

build/azDevOps/azure/azuredevops-vars.yml

@@ -83,7 +83,7 @@ variables:
 
   # DEFAULT IMAGE RUNNER
   - name: pool_vm_image
-    value: ubuntu-20.04
+    value: ubuntu-24.04
 
   # Maven
   - name: maven_package_version