Update stale.yml (#749)

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

Signed-off-by: Luca Comellini <[email protected]>

.github/workflows/stale.yml

@@ -23,7 +23,8 @@ jobs:
           stale-issue-label: "stale"
           stale-pr-label: "stale"
           exempt-all-assignees: true
-          exempt-issue-labels: "proposal"
+          exempt-all-issue-milestones: true
+          exempt-issue-labels: "proposal, backlog, backlog candidate, epic"
           operations-per-run: 100
           days-before-stale: 90
           days-before-close: 10