Gradle plugin 4.2.0 is the last version that supports Java 1.8. For
newer versions RubyMotion would need to support Java 11.

The jcenter repository is deprecated:

   https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

Users are advised to migrate to other repositories.

Also add the default google repository.

The latest version of RubyMotion (8.1) uses Android version 30.
API version 30 is also the minimum version required by the Google Play
Store.

The `--build-file` option has been replaced by `--project-root`.

The `duplicatesStrategy` has to be explicitly set:
https://docs.gradle.org/current/userguide/upgrading_version_5.html#implicit_duplicate_strategy_for_copy_or_archive_tasks_has_been_deprecated

`compile` has been deprecated and should be replaced with `implementation`.

Redefining GRADLE_ROOT will output a warning:

    spec/gradle_spec.rb:14: warning: already initialized constant Motion::Project::Gradle::GRADLE_ROOT

Defining a aidl doesn't seem to work with current versions of gradle.