SuperSorter: Store readOnly output channels. (#16928)

Without the call to readOnly, each output channel retains a 1 MB allocator,
leading to excessive memory use. Fixes regression from #16775.

processing/src/main/java/org/apache/druid/frame/processor/SuperSorter.java

@@ -670,7 +670,7 @@ private void runMerger(
         final OutputChannel outputChannel = outputChannelFactory.openChannel(intRank);
         writableChannel = outputChannel.getWritableChannel();
         frameAllocatorFactory = new SingleMemoryAllocatorFactory(outputChannel.getFrameMemoryAllocator());
-        outputChannels.set(intRank, outputChannel);
+        outputChannels.set(intRank, outputChannel.readOnly());
 
         if (totalMergingLevels == 1) {
           // Reading from the inputBuffer. (i.e. "direct mode"; see class-level javadoc for more details.)