Adds the stale PR workflow (#2593)

This change implements the stale PR template.

## Who is this for?

Developers who want good visibility of work in progress and a process for preventing proliferation of unmerged PRs.

## What is it doing for them?

Closing old PRs that haven't been merged in 30 days. Things that hang about for longer than that should be smaller PRs, released behind a toggle, or be Issues from which other PRs hang.

.github/workflows/stale.yml

@@ -0,0 +1,38 @@
+# Automatically mark any pull requests that have been inactive for 30 days as "Stale"
+# then close them 3 days later if there is still no activity.
+# 
+# Shamelessly copied from https://github.com/guardian/.github/blob/main/workflow-templates/stale.yml
+name: "Stale PR Handler"
+
+on:
+  schedule:
+    # Check for Stale PRs every Monday to Thursday morning
+    # Don't check on Fridays as it wouldn't be very nice to have a bot mark your PR as Stale on Friday and then close it on Monday morning!
+    - cron: "0 6 * * MON-THU"
+
+permissions:
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        id: stale
+        # Read about options here: https://github.com/actions/stale#all-options
+        with:
+          # never automatically mark issues as stale
+          days-before-issue-stale: -1
+
+          # Wait 30 days before marking a PR as stale
+          days-before-stale: 30
+          stale-pr-message: >
+            This PR is stale because it has been open 30 days with no activity.
+            Unless a comment is added or the “stale” label removed, this will be closed in 3 days
+
+          # Wait 3 days after a PR has been marked as stale before closing
+          days-before-close: 3
+          close-pr-message: This PR was closed because it has been stalled for 3 days with no activity.
+
+          # Ignore PR's raised by Dependabot
+          exempt-pr-labels: "dependencies"