Per this https://jenkins.io/blog/2017/04/10/jenkins-has-upgraded-to-java-8/ jenkins now requires java 8.
Also ssh slaves requires java 8 now too. So any security updates to the plugin we won't get it as we doint have java 8. The plugin may refuse to work.
We could use this openjdk 8 ppa for trusty https://launchpad.net/~openjdk-r/+archive/ubuntu/ppa