Upgrade bundled Tomcat in branch-2

proposed by  jojochuang in mailing list:

We migrated from Tomcat to Jetty in Hadoop3, because Tomcat 6 went EOL in
2016. But we did not realize three years after Tomcat 6's EOL, a majority
of Hadoop users are still in Hadoop 2, and it looks like Hadoop 2 will stay
alive for another few years.

Backporting Jetty to Hadoop2 is probably too big of an imcompatibility.
How about migrating to Tomcat9?