fix: update branch filter regex to exclude cla-signatures branch (#8320)

Author: sestinj

.github/workflows/delete-stale-branches.yaml

@@ -22,4 +22,4 @@ jobs:
           pr-check: true
           dry-run: true
           ignore-issue-interaction: true
-          branches-filter-regex: "cla-signatures"
+          branches-filter-regex: "^(?!cla-signatures$).*"