fix for purge old container (radius-project#7616)

# Description

Fix for radius-project#7589

## Type of change

- This pull request fixes a bug in Radius and has an approved issue
(issue link required).

Fixes: radius-project#7589

.github/workflows/purge-test-resources.yaml

@@ -46,6 +46,7 @@ jobs:
           account-type: org
           org-name: radius-project
           token: ${{ secrets.GH_RAD_CI_BOT_PAT }}
+          keep-at-least: 1
   purge_azure_resources:
     name: Azure resources clean-ups
     runs-on: ubuntu-latest