Use self-hosted runner for stale PRs workflow (#1599)

## Changes

The "Close stale PRs" scheduled workflow has been failing because
GitHub-hosted runners' IPs are not in the `databricks` org's IP allow
list ([recent
failure](https://github.com/databricks/databricks-sdk-go/actions/runs/23624888985)).

Switch to the `databricks-deco-testing-runner-group` self-hosted runner
group, consistent with every other workflow in this repo.

## Tests

NO_CHANGELOG=true

Signed-off-by: Ubuntu <renaud.hartert@databricks.com>

Author: renaudhartert-db

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

@@ -6,7 +6,8 @@ on:
 
 jobs:
   stale:
-    runs-on: ubuntu-latest
+    runs-on:
+      group: databricks-deco-testing-runner-group
     permissions:
       pull-requests: write
     steps: