chore: downgrade backpressure warning to a debug log message (#3392)

Until we're more optimized this warning is not terribly useful or
actionable by users.

rust/lance-io/src/scheduler.rs

@@ -222,7 +222,7 @@ impl IoQueueState {
             || since_last_warn > BACKPRESSURE_DEBOUNCE
         {
             tracing::event!(tracing::Level::WARN, "Backpressure throttle exceeded");
-            log::warn!("Backpressure throttle is full, I/O will pause until buffer is drained.  Max I/O bandwidth will not be achieved because CPU is falling behind");
+            log::debug!("Backpressure throttle is full, I/O will pause until buffer is drained.  Max I/O bandwidth will not be achieved because CPU is falling behind");
             self.last_warn
                 .store(seconds_elapsed.max(1), Ordering::Release);
         }