RFR: JDK-8242863: Bump minimum boot jdk to JDK 14

[Erik Joelsson]

JDK 14 is now GA. The minimum boot JDK version for mainline/JDK 15 
should be bumped to this version.

Bug: https://bugs.openjdk.java.net/browse/JDK-8242863
Webrev: http://cr.openjdk.java.net/~erikj/8242863/webrev.01/index.html

/Erik