test: fix test_pageserver_recovery flakyness (#7207)

## Problem
We recently introduced log file validation for the storage controller.
The heartbeater will WARN when it fails
for a node, hence the test fails.

Closes https://github.com/neondatabase/neon/issues/7159

## Summary of changes
* Warn only once for each set of heartbeat retries
* Allow list heartbeat warns

control_plane/attachment_service/src/heartbeater.rs

@@ -139,7 +139,7 @@ impl HeartbeaterTask {
                         .with_client_retries(
                             |client| async move { client.get_utilization().await },
                             &jwt_token,
-                            2,
+                            3,
                             3,
                             Duration::from_secs(1),
                             &cancel,