fix: replace invalid return with exit 0 in backport workflow (#5401)

Fixes shell script error where 'return' was used outside of a function.
In shell scripts, 'exit 0' should be used to exit with success status.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/backport.yaml

@@ -36,7 +36,7 @@ jobs:
           
           if [ -z "$EXISTING_BACKPORTS" ]; then
             echo "skip=false" >> $GITHUB_OUTPUT
-            return
+            exit 0
           fi
           
           echo "Found existing backport PRs:"