Remove mavenBom import

[jdaugherty]

Since we now have changed the projects to not export the bom via api, we should enforce the bom be imported with platform("org.grails:grails-bom:${grailsVersion}") and not perform this automatically.

Grails-core now includes functional tests, and this import is causing the dependency substitution not to work.

[jdaugherty]

Error from publishing run:

    Cannot resolve reloading agent JAR: Failed to resolve imported Maven boms: Could not find org.grails:grails-bom:7.0.0-M2.
    Required by:
     project :grails-test-examples-app3

[jdaugherty]

This can't be removed until post Grails 7. We'll have to take a strategy of:

1. Internal Projects - use gradle platforms
2. Grails Project - use dependency management and have the gradle plugin apply the mavenBom