Document how to migrate usage of EnableBatchProcessing(modular = true) to v6

[fmbenhassine]

This is related to #4866. The goal is to document how to migrate the use of EnableBatchProcessing(modular = true) to Spring's context hierarchies and GroupAwareJobs (as mentioned in the javadoc of EnableBatchProcessing).

cc @kzander91 (who provided a starting point in #4866 (comment), thank you for that!)

[fmbenhassine]

Added example here: https://github.com/spring-projects/spring-batch/wiki/Spring-Batch-6.0-Migration-Guide#changes-related-to-the-modular-batch-configurations-through-enablebatchprocessingmodular--true