Raise minimum java version to 1.6 (is 1.5)

[Samuel Mehrbrodt]

Hi,

Currently we build with the 1.5 target so that Java 5 can still be used 
as a JRE in LO.

Java 9 doesn't support 1.5 as a target anymore, the minimum target there 
is 1.6.

Can we raise the target version to 1.6 for LibreOffice 6.3?

Thanks
Samuel