Mark and close stale PRs (#278)

Borrowed from https://github.com/open-telemetry/opentelemetry-specification/blob/main/.github/workflows/stale-pr.yaml

Author: tigrannajaryan

.github/workflows/stale-pr.yaml

@@ -0,0 +1,25 @@
+name: "Close stale pull requests"
+on:
+  schedule:
+    - cron: "10 4 * * *" # arbitrary time not to DDOS GitHub
+
+permissions:
+  contents: read
+
+jobs:
+  stale:
+    permissions:
+      pull-requests: write # required for marking and closing 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. It will be closed in 30 days without additional activity.'
+          close-pr-message: 'Closed as inactive. Feel free to reopen if this PR is still being worked on.'
+          # opt out of defaults to avoid marking issues as stale
+          days-before-stale: -1
+          days-before-close: -1
+          # overrides the above only for pull requests
+          days-before-pr-stale: 30
+          days-before-pr-close: 14