Tiered compaction: fix early exit check in main loop (#7702)

The old test based on the immutable `target_file_size` that was a
parameter to the function.

It makes no sense to go further once `current_level_target_height` has
reached `u64::MAX`, as lsn's are u64 typed. In practice, we should only
run into this if there is a bug, as the practical lsn range usually ends
much earlier.

Testing on `target_file_size` makes less sense, it basically implements
an invocation mode that turns off the looping and only runs one
iteration of it.
@hlinnaka agrees that `current_level_target_height` is better here.

Part of #7554

pageserver/compaction/src/compact_tiered.rs

@@ -106,7 +106,13 @@ pub async fn compact_tiered<E: CompactionJobExecutor>(
             ctx,
         )
         .await?;
-        if target_file_size == u64::MAX {
+        if current_level_target_height == u64::MAX {
+            // our target height includes all possible lsns
+            info!(
+                level = current_level_no,
+                depth = depth,
+                "compaction loop reached max current_level_target_height"
+            );
             break;
         }
         current_level_no += 1;