[11] Review request: 8200446: Update minimum boot JDK to 10

[Kevin Rushforth]

Please review the following to bump the minimum boot JDK required for 
JavaFX builds to JDK 10. Note that the review will be done on github, 
but it is OK to reply to this thread with comments if you don't have a 
github account.

https://bugs.openjdk.java.net/browse/JDK-8200446
https://github.com/javafxports/openjdk-jfx/pull/61

This also bumps the class file format to that of JDK 10. I note that we 
have been using JDK 10 to build the openjfx-11 builds for several 
months, and now that we've upgraded to gradle 4.8 we are ready to make 
JDK 10 the minimum.

Once this is done, we can start using JDK 10 features, such as 'var' for 
local variables (although please don't take this as a license to use 
'var' gratuitously).

Thanks.

-- Kevin