RFR: JDK-8221414: Bump required boot jdk version to 12

[Erik Joelsson]

With the release of JDK 12 it is now time to update the required boot 
jdk version for JDK 13 (to 12). This patch changes the configure 
requirements as well as bumps the version used for Jib builds at Oracle.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221414
Webrev: http://cr.openjdk.java.net/~erikj/8221414/webrev.01/

/Erik