RFR: 8240542: Switch FX build to use JDK 14 as boot JDK

[Kevin Rushforth]

On Fri, 27 Mar 2020 21:17:04 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> Marked as reviewed by jvos (Reviewer).
>
>> We will continue to generate class files using `-source 11 -target 11`
> 
> Isn't `-release 11` the preferred way?

gradle's java plugin translates `sourceCompatibility NN` into `-source NN -target NN`. I haven't looked into whether
gradle has a way to use `-release`. In practice, what it means is that we need to be careful not to use newer API from
a newer JDK.

We could look into changing it if gradle provides that support, but that would be a separate issue, independent from
upgrading the boot JDK from 13 to 14.

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PR: https://git.openjdk.java.net/jfx/pull/152