Drop support for Java 6

[eamonnmcmanus]

Continuing to support Java 6 causes considerable headaches. Building with -source 6 -target 6 is not supported in JDK versions after 11. I don't think we need to support further back than Java 7 even for the most ancient Android devices still in use.

[Marcono1234]

I don't think we need to support further back than Java 7 even for the most ancient Android devices still in use.

Would it make sense to directly move to Java 8? Then Gson could in the future easily add support for Optional (#1102) and the java.time package (#1059), possibly by integrating https://github.com/google-gson/typeadapters.

[eamonnmcmanus]

I think we'll want Gson to be like Guava. Guava supports Java 7 for Android. In reality, Android tooling does support Java 8 language features such as lambdas even for old Android versions, but those old versions don't have Java 8 API features such as java.util.Optional and java.time. If we wanted to support those, I think we would either have to use reflection or have separate artifacts for Java 7 and Java 8.

At some point, Guava will probably stop supporting these old versions and then Gson can follow suit.