Separate Gradle getting started from the Maven one

(Creating the issue here so that it gets some visibility for new contributors)

Currently, the getting started project for Gradle in https://github.com/quarkusio/quarkus-quickstarts/ is the same as the Maven getting started.

This is a mistake as the project structure should be different (especially the unit tests and the integration tests should be in 2 different folders).

We need to create a specific project for Gradle and get rid of the Gradle files in the getting-started project.

The project should be built with the Gradle command line.

Note to self: we will need to upgrade the release scripts once it's done.