Use the right parameter for milestones (#1542)

Problem: `exempt-issue-milestones` takes a list of milestones to
exclude.

Solution: `exempt-all-issue-milestones` excludes issues that have any milestones
attached.

.github/workflows/stale.yml

@@ -22,7 +22,7 @@ jobs:
           close-pr-message: "This PR was closed because it has been stalled for 14 days with no activity."
           stale-issue-label: "stale"
           stale-pr-label: "stale"
-          exempt-issue-milestones: true
+          exempt-all-issue-milestones: true
           exempt-issue-labels: "backlog, backlog candidate"
           operations-per-run: 100
           days-before-stale: 14