Deprecate modular job configuration through EnableBatchProcessing

[fmbenhassine]

The support of job configuration modularity through @EnableBatchProcessing(modular = true) to avoid job name collisions in the JobRegistry requires an unnecessary amount of accidental complexity both in terms of APIs the user has to provide as well as implementation details. The amount of classes and interfaces needed for this feature (JobFactory, ApplicationContextFactory, ApplicationContextJobFactory, ReferenceJobFactory, AbstractApplicationContextFactory (with its two extensions), JobLoader, JobFactoryRegistrationListener and others) does not justify the real gain. This feature could be achieved by leveraging GroupAwareJobs or by using a simple naming convention like namespace.jobName.

This feature should be deprecated in v6 and removed in 6.2 or later.