Add workflow for managing stale issues & PRs (#1055)

Adds a GitHub workflow for managing stale & rotten issues and PRs. 

Issues & PRs are marked stale after 90 days of inactivity. After 60 further days, they are marked as rotten and closed.

Author: johnmcollier

Diff for: .github/workflows/stale.yaml

@@ -0,0 +1,35 @@
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: "0 0 * * *"
+
+permissions:
+  contents: read
+
+jobs:
+  stale:
+
+    permissions:
+      issues: write  # for actions/stale to close stale issues
+      pull-requests: write  # for actions/stale to close stale PRs
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/stale@v7
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-issue-message: "This issue is stale because it has been open for 90 days with no activity. Remove stale label or comment or this will be closed in 60 days."
+        stale-pr-message: "This PR is stale because it has been open for 90 days with no activity. Remove stale label or comment or this will be closed in 60 days."
+
+        days-before-stale: 90
+        days-before-close: 60
+        stale-issue-label: 'lifecycle/stale'
+        stale-pr-label: 'lifecycle/stale'
+        exempt-issue-labels: 'lifecycle/frozen'
+        exempt-pr-labels: 'lifecycle/frozen'
+        close-issue-label: 'lifecycle/rotten'
+        close-pr-label: 'lifecycle/rotten'
+
+        start-date: '2023-03-16T00:00:00Z'
+