Switched to GitHub Action for handling stale issues + PRs

refs https://github.com/TryGhost/Toolbox/issues/81

- https://github.com/actions/stale is the official way to handle stale
  issues and PRs and our old method hasn't worked for a while
- most of this is copied from [Ghost-CLI's stale
  workflow](https://github.com/TryGhost/Ghost-CLI/blob/main/.github/workflows/stale.yml)
  but the comment contents and labels have been updated
- also removes the config for the broken, existing method

.github/stale.yml

@@ -1,17 +0,0 @@
-# Number of days of inactivity before an issue becomes stale
-daysUntilStale: 90
-# Number of days of inactivity before a stale issue is closed
-daysUntilClose: 7
-# Issues with these labels will never be considered stale
-exemptLabels:
-  - pinned
-  - feature
-# Label to use when marking an issue as stale
-staleLabel: stale
-# Comment to post when marking an issue as stale. Set to `false` to disable
-markComment: >
-  This issue has been automatically marked as stale because it has not had
-  recent activity. It will be closed if no further activity occurs. Thank you
-  for your contributions.
-# Comment to post when closing a stale issue. Set to `false` to disable
-closeComment: false

.github/workflows/stale.yml

@@ -0,0 +1,25 @@
+name: 'Close stale issues and PRs'
+on:
+  schedule:
+    - cron: '0 0 * * *'
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v3
+        with:
+          stale-issue-message: |
+            Our bot has automatically marked this issue as stale because there has not been any activity here in some time.
+
+            The issue will be closed soon if there are no further updates, however we ask that you do not post comments to keep the issue open if you are not actively working on a PR.
+
+            We keep the issue list minimal so we can keep focus on the most pressing issues. Closed issues can always be reopened if a new contributor is found. Thank you for understanding 🙂
+          stale-pr-message: |
+            Our bot has automatically marked this PR as stale because there has not been any activity here in some time.
+
+            If we’ve missed reviewing your PR & you’re still interested in working on it, please let us know. Otherwise this PR will be closed shortly, but can always be reopened later. Thank you for understanding 🙂
+          exempt-issue-labels: 'feature,pinned'
+          exempt-pr-labels: 'feature,pinned'
+          days-before-stale: 120
+          stale-issue-label: 'stale'
+          stale-pr-label: 'stale'