chore: fix stale workflow (#1228)

From `actions/stale@v2` the option for exempt labels
was renamed to `exempt-issue-labels` (plural). Also
it appears that this repository is using `never-stale`
(with a hyphen instead of a space character).

Refs: actions/stale#19

.github/workflows/stale.yml

@@ -12,7 +12,7 @@ jobs:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         stale-issue-message: 'This issue is stale because it has been open many days with no activity. It will be closed soon unless the stale label is removed or a comment is made.'
         stale-issue-label: 'stale'
-        exempt-issue-label: 'never stale'
+        exempt-issue-labels: 'never-stale'
         days-before-stale: 90
         days-before-close: 30