Revert jdk-12 to jdk-8

[mishaschwartz]

• if you want to use jdk-12 you can update manually after the java/jdbc install or you can change the install script for the java/jdbc testers before running yourself
• Eventually when installing jdk-12 by default makes more sense/is easier to do we can do that (see update jdk version from jdk-8 #197)

[david-yz-liu]

@mishaschwartz If only available on 18.04+ then please leave this at version 8 for now. I plan to merge this in soon (so that it fixes my build).

[mishaschwartz]

@david-yz-liu I'm just about to update the PR on the Markus side to update the vagrant machine to 18.04 as we discussed.

[david-yz-liu]

@mishaschwartz I realized, we should probably wait to upgrade to JDK 11/12 anyway until the teaching labs do.

[mishaschwartz]

@david-yz-liu it is currently being discussed since we are upgrading everything to 18.04 this semester. It would be good to have a jdk-11 in development at least so we can catch any issues before we have to use jdk-11 in production.