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.

Vom Indexierungsmodell aus dem Issue-Text verfasst.