HDFS-17971. Fix RAT check failure for vendored JSON.java in hadoop-hdfs-rbf. (#8708) Contributed by Shilun Fan

* HDFS-17971. Fix RAT check failure for vendored JSON.java in hadoop-hdfs-rbf.

Exclude the vendored JSON.java source from the Apache RAT scan. The file retains its upstream EPL-1.0/Apache-2.0 dual-license header, which RAT does not recognize as the standard ASF license header. 
This avoids the false positive without modifying the original licensing notice.

Reviewed-by: Hualong Zhang <hualong.z@hotmail.com>
Signed-off-by: Shilun Fan <slfan1989@apache.org>