MINOR: Supress stdout when checking Log4j 1.x configuration compatibility mode (#19502)

when using log41 config, we are printing addtional line like below. This
PR is to fix that.

Author: omkreddy

Diff for: bin/kafka-run-class.sh

@@ -225,7 +225,7 @@ if [ -z "$KAFKA_LOG4J_OPTS" ]; then
   (( WINDOWS_OS_FORMAT )) && LOG4J_DIR=$(cygpath --path --mixed "${LOG4J_DIR}")
   KAFKA_LOG4J_OPTS="-Dlog4j2.configurationFile=${LOG4J_DIR}"
 else
-  if echo "$KAFKA_LOG4J_OPTS" | grep -E "log4j\.[^[:space:]]+(\.properties|\.xml)$"; then
+  if echo "$KAFKA_LOG4J_OPTS" | grep -E "log4j\.[^[:space:]]+(\.properties|\.xml)$" >/dev/null; then
       # Enable Log4j 1.x configuration compatibility mode for Log4j 2
       export LOG4J_COMPATIBILITY=true
       echo DEPRECATED: A Log4j 1.x configuration file has been detected, which is no longer recommended. >&2