This updates the Gradle version needed to build the Java bindings
from 2.7 to 6.7. A lot of time elapsed between these two releases:
the build system has been rewritten from scratch using the Kotlin
DSL.

The build system now relies on the new C++ plugins, which will
eventually replace the software model plugins and take advantage of
many new features baked into Gradle core.

Java 11 can be used to build the bindings.