Don't require partial backup semaphore capacity for deletions (#10628)

In the safekeeper, we block deletions on the timeline's gate closing,
and any `WalResidentTimeline` keeps the gate open (because it owns a
gate lock object). Thus, unless the `main_task` function of a partial
backup doesn't return, we can't delete the associated timeline.

In order to make these tasks exit early, we call the cancellation token
of the timeline upon its shutdown. However, the partial backup task
wasn't looking for the cancellation while waiting to acquire a partial
backup permit.

On a staging safekeeper we have been in a situation in the past where
the semaphore was already empty for a duration of many hours, rendering
all attempted deletions unable to proceed until a restart where the
semaphore was reset:
https://neondb.slack.com/archives/C03H1K0PGKH/p1738416586442029

safekeeper/src/wal_backup_partial.rs

@@ -535,6 +535,10 @@ pub async fn main_task(
         // limit concurrent uploads
         let _upload_permit = tokio::select! {
             acq = limiter.acquire_partial_backup() => acq,
+            _ = backup.tli.cancel.cancelled() => {
+                info!("timeline canceled");
+                return None;
+            }
             _ = cancel.cancelled() => {
                 info!("task canceled");
                 return None;