Document how to use Gradle's native bom support as an alternative to the dependency management plugin

[asaikali]

After watching Managing Dependencies for Spring Projects with Gradle by Jenn Strater and Andy Wilkinson talk. I started a new project using the Gradle Java Platfrom Plugin rather than the Spring Boot dependency gradle plugin which worked but required a lot of trial and error. Upgrading from boot 2.2.6 to 2.3.0 caused the breakage #21569.

It's unclear how the boot plugin interacts with the Java Platform Plugin. Documenting how to use the Java Platform Plugin rather than the dependency management plugin is quite useful for many users.

[philwebb]

We should also add a smoke test if it's brittle.

[wilkinsona]

The Java Platform Plugin isn't really an alternative to the Dependency Management Plugin. The former is used when you're creating a project that defines a platform/bom. The latter is used when you're consuming a bom. A more typical replacement for the dependency management plugin is a platform or enforcedPlatform dependency on spring-boot-dependencies. I think that's what we should document.