workflows: Lock issues that have been closed for 6+ months

Add a new automation to lock closed issues with no recent updates.
This is intended to reduce updates to old tickets that few people will
see.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

.github/workflows/stale-issue-bot.yaml

@@ -194,3 +194,14 @@ jobs:
                 state: 'closed'
               });
             }
+  # Lock closed issues after 6 months of inactivity.
+  lock:
+    name: Lock Closed Issues
+    if: github.repository == 'Klipper3d/klipper'
+    runs-on: ubuntu-latest
+    steps:
+    - uses: dessant/lock-threads@v3
+      with:
+        process-only: 'issues'
+        issue-inactive-days: '180'
+        issue-lock-reason: ''