Bug#894290: gradle should raise the source/target level to the minimum supported by the default JDK

[Emmanuel Bourg]

Control: severity -1 wishlist

I'm downgrading the severity because this isn't actionable for now
(implementation suggestions are welcome) and we need
gradle-debian-helper 2.0 and gradle 4.4 in testing. The source/target
level of the affected packages should be updated separately.