HEADS-UP: Updating minimum boot JDK to 11 for OpenJFX 12

[Kevin Rushforth]

As a heads-up, the following issue is in progress for OpenJFX 12:

https://bugs.openjdk.java.net/browse/JDK-8209966
https://github.com/javafxports/openjdk-jfx/pull/174

This will bump the minimum boot JDK version to JDK 11 for building and 
testing OpenJFX 12. Note that this will *not* affect OpenJFX 11 in any way.

This will enable a few follow-on issues to be done:

1. Stop redistributing Microsoft DLLs :: JDK-8210089 [1]
2. Remove old javafx.swing implementation :: JDK-8210092 [2]
3. Build with --source 11 and --target 11 :: JDK-8210093 [3]

as well making some additional cleanup of the build possible.

Before sending it out for formal review, I wanted to give everyone a 
heads-up that this is coming, so you can prepare for it.

For the timing of this, my plan was to wait until the final version of 
JDK 11 is released. We probably could do it a week or two ahead of the 
final release to unblock work on the follow-on issues, depending on what 
the rest of the people who build OpenJFX from source think.

-- Kevin

[1] https://bugs.openjdk.java.net/browse/JDK-8210089
[2] https://bugs.openjdk.java.net/browse/JDK-8210092
[3] https://bugs.openjdk.java.net/browse/JDK-8210093