Proposal: Bump minimum JDK version for JavaFX 20 to JDK 17

[Kevin Rushforth]

Even though we build JavaFX binaries with JDK 18 as the boot JDK, the 
latest version of JavaFX still runs with JDK 11 (and is capable of being 
built using JDK 12 or later, and with some limitations, using JDK 11), 
although it isn't tested with older JDK versions. In order for JavaFX to 
be able to use newer JDK features, such as records, switch expressions, 
text blocks, and so forth, we need to increase the minimum version of 
the JDK that can run the latest JavaFX. Additionally, there is an 
ongoing cost to keeping JavaFX buildable and runnable on older versions 
of Java, and very little reason to continue to do so.

To this end, I propose to bump the minimum version of the JDK needed to 
run JavaFX 20 to JDK 17. I filed JDK-8290530 [1] to track this. This 
will not affect update releases of earlier versions of JavaFX (e.g., 
JavaFX 17.0.NN), which will continue to run with the same minimum JDK 
that they run on today.

As a reminder, we only assure that JavaFX NN will run with JDK NN-1 or 
later, although in practice, we haven't bumped the minimum required JDK 
version in several releases. So, while JavaFX 19 is built using JDK 18 
as the boot JDK, it produces class files that will run with JDK 11, 
using "--source 11 --target 11". The proposed change discussed here 
would update that in JavaFX 20 to "--source 17 --target 17".

NOTE: this will not be an invitation to do wholesale refactoring of 
existing classes or methods to use newer language features (e.g., a PR 
that turns a bunch of existing data classes into records would not be 
welcome). Rather, this can be seen as enabling judicious use of new 
features in new code, much as we did when we started allowing the use of 
"var".

Absent a compelling reason to remain stuck in the past, I plan to send 
out a pull request for this change next week.

Comments are welcome.

-- Kevin

[1] https://bugs.openjdk.org/browse/JDK-8290530