Update Ubuntu version for cache dependencies pipeline (#5596)

Co-authored-by: Liu Zhe <zhe.liu@microsoft.com>

pipelines/cache-dependencies.yml

@@ -17,7 +17,7 @@ jobs:
 
 - job: ubuntu_legacy
   pool:
-    vmImage: ubuntu-18.04
+    vmImage: ubuntu-20.04
   steps:
   - template: templates/cache-dependencies-template.yml
     parameters: