always notify gc_waiting when writing over the witness tracking

pageserver/src/tenant/timeline/detach_ancestor.rs

@@ -146,6 +146,7 @@ impl SharedState {
             .lock()
             .unwrap()
             .continue_existing_attempt(attempt);
+        self.gc_waiting.notify_one();
     }
 
     /// Only GC must be paused while a detach ancestor is ongoing. Compaction can happen, to aid
@@ -513,6 +514,7 @@ impl SharedStateBuilder {
         g.known_ongoing.extend(self.inprogress);
         if g.latest.is_none() && !g.known_ongoing.is_empty() {
             g.latest = Some((ExistingAttempt::ReadFromIndexPart, false));
+            target.gc_waiting.notify_one();
         }
     }
 }