Deprecate XML-DSL

In order to improve the maintainability of the framework and reduce the complexity from a user’s perspective, we are going to deprecate the XML-DSL. The use case for the XML-DSL was to provide a way to create integration tests without any knowledge of Java. As we are moving to a DevOps world and software developers become testers as well, the use case for the XML-DSL gets more and more obsolete. The deprecation should encourage users to start developing their tests in Java and migrate their current XML-DSL test suites. It will be some time before the XML-DSL is completely removed from the framework but going forward the focus is clearly on the Java DSL. We will announce the final removal with a lead time of at least one year to give users enough time for migration. Until then, the XML-DSL will still be usable as it is but it will not be extended or supported as part of the open source project.

- [ ] https://github.com/citrusframework/citrus/issues/607
- [ ] https://github.com/citrusframework/citrus/issues/608
- [ ] https://github.com/citrusframework/citrus-samples/issues/41