[12] RFR: JDK-8209966: Update minimum boot JDK to 11

[Kevin Rushforth]

Please review the following on GitHub:

https://bugs.openjdk.java.net/browse/JDK-8209967
https://github.com/javafxports/openjdk-jfx/pull/169

This bumps the minimum version of gradle for openjfx 12 to the current 
4.8 version (which is the version used by our builds and by gradlew). 
This is needed in preparation for eventually bumping the minimum boot 
JDK to 11 after JDK 11 ships.

-- Kevin