storcon: sk heartbeat fixes (#10891)

This PR does the following things:

* The initial heartbeat round blocks the storage controller from
becoming online again. If all safekeepers are unresponsive, this can
cause storage controller startup to be very slow. The original intent of
#10583 was that heartbeats don't affect normal functionality of the
storage controller. So add a short timeout to prevent it from impeding
storcon functionality.

* Fix the URL of the utilization endpoint.

* Don't send heartbeats to safekeepers which are decomissioned.

Part of https://github.com/neondatabase/neon/issues/9011

context: https://neondb.slack.com/archives/C033RQ5SPDH/p1739966807592589

safekeeper/client/src/mgmt_api.rs

@@ -137,7 +137,7 @@ impl Client {
     }
 
     pub async fn utilization(&self) -> Result<SafekeeperUtilization> {
-        let uri = format!("{}/v1/utilization/", self.mgmt_api_endpoint);
+        let uri = format!("{}/v1/utilization", self.mgmt_api_endpoint);
         let resp = self.get(&uri).await?;
         resp.json().await.map_err(Error::ReceiveBody)
     }