docs: clarify Jenkins BOM usage for Pipeline testing

[ayushHardeniya]

While reviewing the developer testing documentation, I noticed that the Pipeline testing section
references the Jenkins Bill of Materials (BOM) without explaining how it is used in practice.

This change adds a brief explanation and a concrete Maven dependencyManagement example to clarify
the recommended approach for managing test dependency versions.

[MarkEWaite]

This seems like it is duplicating the instructions in the linked page and making a mistake while doing it.

If the link is not enough to explain the plugin bill of materials, then I think we should provide a link to the "Use the plugin BOM" section of the "Improve a plugin" tutorial.

[ayushHardeniya]

Thanks for the clarification.
I’ve removed the inline BOM example and updated the section to link directly to the “Use the plugin BOM” documentation in the Improve a plugin tutorial instead, to avoid duplication and ensure correctness.