chore: revert aws-actions/stale-issue-cleanup back to v6 (#853)

There are some weird issues with the latest version. Let's just go back
to the tried and tested on.


https://github.com/aws/aws-cdk-cli/actions/runs/17833245041/job/50705475999

---
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under the terms of the Apache-2.0 license

Author: mrgrain

.github/workflows/close-stale-issues.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     name: Stale issue job
     steps:
-    - uses: aws-actions/stale-issue-cleanup@v7.1.0
+    - uses: aws-actions/stale-issue-cleanup@v6
       with:
         # Setting messages to an empty string will cause the automation to skip
         # that category