[Spring] Deprecate cucumber.xml and implied context configuration

[mpkorstanje]

The prefered way to use cucumber-spring is to annotate a class
with both @CucumberContextConfiguration and a Spring context
configuration annotation such as @ContextConfiguration,
@SpringBootTest, ect.

Cucumber currently supports the discovery of context
configuration on any step defintion class. This requires a
significant amount of code and intimate knowledge of the
Spring Framework.
By restricting this too only classes annotated with
@CucumberContextConfiguration we can remove this complexity and
simply pass the annotated class to Spring TestContextManager
framework.

If no context configuration is available Cucumber currently also
support reading the application context from a magical
cucumber.xml file or should none exist an empty application
context.
Both these fallback strategies tend to hide user errors. And
removing them will provide more clarity and also remove
a nuggest of complexity.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage decreased (-0.05%) to 86.105% when pulling e12c7fc on spring-deprecate-and-update into e544745 on master.