Update pr-stale.yml (#1301)

Author: Pier-Luc Caron St-Pierre

.github/workflows/pr-stale.yml

@@ -13,6 +13,7 @@ jobs:
         operations-per-run: 100
         days-before-stale: 15
         days-before-close: 7
+        exempt-all-assignees: true # anything with a assignee won't be closed
         stale-pr-message: "This PR is more than 2 weeks old. Please remove the
           stale label, update, or comment if this PR is still valid and
           relevant, otherwise it will be closed in 7 days."