[exec][email] Java 7 to 8

[Gary Gregory]

I do not see a reason to maintain EXEC and EMAIL on Java 7 at this point,
it's simpler to maintain Commons builds locally, on GitHub Actions, and
Travis CI by using Java 8.

FYI, DAEMON is still on Java 6, presumably to support Tomcat. I will start
a separate thread about that, just to check status.

Gary