chore(ci): migrate probot-stale to actions/stale

Signed-off-by: Dominic Evans <dominic.evans@uk.ibm.com>

.github/workflows/stale.yml

@@ -0,0 +1,36 @@
+# configuration for https://github.com/actions/stale
+name: "Stale issues and PRs"
+on:
+  schedule:
+    - cron: "0 */2 * * *"
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/stale@v8
+        with:
+          days-before-stale: 90
+          days-before-close: 30
+          stale-issue-message: >-
+            Thank you for taking the time to raise this issue. However, it has
+            not had any activity on it in the past 90 days and will be closed in
+            30 days if no updates occur.
+
+            Please check if the main branch has already resolved the issue
+            since it was raised. If you believe the issue is still valid and you
+            would like input from the maintainers then please comment to ask for
+            it to be reviewed.
+          stale-pr-message: >-
+            Thank you for your contribution! However, this pull request has not
+            had any activity in the past 90 days and will be closed in 30 days if
+            no updates occur.
+
+            If you believe the changes are still valid then please verify your
+            branch has no conflicts with main and rebase if needed. If you
+            are awaiting a (re-)review then please let us know.
+          stale-issue-label: "stale"
+          exempt-issue-label: "stale/exempt,pinned"
+          stale-pr-label: "stale"
+          exempt-pr-label: "stale/exempt,pinned"