Update stale.yml (#302)

Avoid closing issues that have a milestone or with backlog/epic labels

Author: lucacome

.github/workflows/stale.yml

@@ -22,5 +22,7 @@ jobs:
           close-pr-message: "This PR was closed because it has been stalled for 7 days with no activity."
           stale-issue-label: "stale"
           stale-pr-label: "stale"
+          exempt-all-issue-milestones: true
+          exempt-issue-labels: "backlog, backlog candidate, epic"
           exempt-all-pr-assignees: true
           operations-per-run: 100