MINOR: Rat should ignore generated directories (#7729)
MINOR: Rat should ignore generated directories (#7729)

For some reason, PR builds are failing due to the `rat` license
check even though it should ignore files included in `.gitignore`.

Reviewers: Jason Gustafson <68c46a606457643eab92053c1c05574abb26f861@confluent.io>
lindong28/kafka,noslowerdna/kafka,Chasego/kafka,guozhangwang/kafka,guozhangwang/kafka,TiVo/kafka,TiVo/kafka,sslavic/kafka,sslavic/kafka,apache/kafka,apache/kafka,lindong28/kafka,lindong28/kafka,noslowerdna/kafka,guozhangwang/kafka,noslowerdna/kafka,TiVo/kafka,lindong28/kafka,sslavic/kafka,apache/kafka,Chasego/kafka,Chasego/kafka,noslowerdna/kafka,apache/kafka,Chasego/kafka,sslavic/kafka,guozhangwang/kafka,TiVo/kafka