change DROP_WARN message from debug to warn

Author: willyborankin

src/main/java/io/aiven/kafka/connect/transforms/TombstoneHandler.java

@@ -69,7 +69,7 @@ private void behaveOnNull(final ConnectRecord<R> r) {
                 );
                 break;
             case DROP_WARN:
-                LOGGER.debug(
+                LOGGER.warn(
                     "Tombstone record encountered, dropping due to configured '{}' behavior",
                     TombstoneHandlerConfig.Behavior.DROP_WARN.name().toLowerCase()
                 );