k/proto: use log-level warn for invalid crc

Recently, redpanda-data#11324 changed the log-level from error to warn for a
CRC missmatch. However, on the same event, RP will also log "batch has
invalid CRC" at the error level.

This commit therefore changes the log for "batch has invalid CRC" to
warn level.

Fixes redpanda-data#11361

src/v/kafka/protocol/kafka_batch_adapter.cc

@@ -168,7 +168,7 @@ iobuf kafka_batch_adapter::adapt(iobuf&& kbatch) {
 
     verify_crc(header.crc, std::move(crcparser));
     if (unlikely(!valid_crc)) {
-        vlog(klog.error, "batch has invalid CRC: {}", header);
+        vlog(klog.warn, "batch has invalid CRC: {}", header);
         return remainder;
     }