Move JDK baseline to Java 8 #75

(note: follow-up to #74 )

Since it is difficult to even build Java libraries with pre-JDK-8, let's:

1. Create branch 1.6 to allow for theoretical JDK6-compatible builds
2. Update master to 1.7 and update baseline JDK requirement to Java 8

and then start addressing issues like missing Github Action for CI and failures on Java 17+.