JLBP-16 Recommend using Gradle version catalogs

The recommended `ext` extension in root project for Gradle in https://jlbp.dev/JLBP-16 is bad practice nowadays (cross project configuration), and won't work with Gradle's Isolated Projects feature.

Gradle has had a Version Catalog feature for some time now specifically to declare dependency versions to be used across the build.

Start with the root project’s Gradle configuration and the guidance in JLBP-16. Replace the recommended root-project ext dependency version declarations with Gradle version catalogs, then verify that dependency versions remain available across the build and are compatible with Gradle Isolated Projects.

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