storcon: don't log a heartbeat error during shutdown (#10325)

## Problem

Occasionally we see an unexpected error like:
```
ERROR spawn_heartbeat_driver: Failed to update node state 1 after heartbeat round: Shutting down\n')
Hint: use scripts/check_allowed_errors.sh to test any new allowed_error you add
```


https://neon-github-public-dev.s3.amazonaws.com/reports/pr-10324/12690404952/index.html#/testresult/63406a0687bf6eca

## Summary of changes

- Explicitly handle ApiError::ShuttingDown as a no-op when mutating node
status

storage_controller/src/service.rs

@@ -1047,6 +1047,9 @@ impl Service {
                             // on a snapshot of the nodes.
                             tracing::info!("Node {} was not found after heartbeat round", node_id);
                         }
+                        Err(ApiError::ShuttingDown) => {
+                            // No-op: we're shutting down, no need to try and update any nodes' statuses
+                        }
                         Err(err) => {
                             // Transition to active involves reconciling: if a node responds to a heartbeat then
                             // becomes unavailable again, we may get an error here.