CI: close stale PRs (open-telemetry#1043)

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>

Author: florianl

.github/workflows/stale.yml

@@ -0,0 +1,20 @@
+name: "Close stale pull requests"
+on:
+  schedule:
+    - cron: "15 8 * * *"
+
+permissions: read-all
+
+jobs:
+  stale:
+    permissions:
+      pull-requests: write # for actions/stale to close stale PRs
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@997185467fa4f803885201cee163a9f38240193d # v10.1.1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          stale-pr-message: "This PR was marked stale due to lack of activity. It will be closed in 14 days."
+          close-pr-message: "Closed as inactive. Feel free to reopen if this PR is still being worked on."
+          days-before-pr-stale: 28
+          days-before-pr-close: 14