Update Java version requirement to 11

[David E Jones]

The main remaining reason to keep the Java version requirement at Java 8 has been the lack of support for Java 11 on IBM iSeries which some Moqui users still use. It appears that is now available:

April, 2021:  Java 11 GA

IBM Technology for Java 11 is released on IBM i 7.3 and higher. It is released as off-release delivery for product 5770JV1 option 19 (Java 11 64 bit).
Instructions on how to use Java 11 on i:
1. Download Java 11 from the Entitled Software Support(ESS)
2. PTF Group SF99725 level 21 or higher is required for i 7.3;  PTF Group SF99665 level 10 or higher is required for i 7.4.
3. Set JAVA_HOME to /QOpenSys/QIBM/ProdData/JavaVM/jdk11/64bit before invoking the Java.

For context, this is one of the changes planned before the 3.0.0 release to combine another non-backward compatible change.

I have created a branch in the moqui-framework repository to work on this:

https://github.com/moqui/moqui-framework/tree/java11

When using this branch, and after it is merged: make sure Java 11 is installed (such as openjdk-11-jdk or adoptopenjdk-11-openj9 on Linux), active (on Linux use 'sudo update-alternatives --config java' so 'java -version' reports 11), and that JAVA_HOME is set to the Java 11 JDK install path (for openjdk-11-jdk on Linux: /usr/lib/jvm/java-11-openjdk-amd64).

Until changes are made in other repos also note that all component build.gradle files need the sourceCompatibility and targetCompatibility values updated to 11 or commented out.