Remove buildSrc from Grails dependencies

[jdaugherty]

Issue description

buildSrc was adopted on several of the core grails dependencies and has proven to be a maintenance problem when we released the first milestone of Grails 7. After discussion in the stewards meeting, we have decided to remove it from the core dependencies going forward.

Specific problems include:

1. independent version resolution
2. independent BOM application
3. only a few projects have classes in buildSrc and we have gradle plugins that can be distributed across projects:
   https://github.com/grails/grails-doc/tree/7.0.x/buildSrc/src/main/groovy/grails/doc
   https://github.com/grails/grails-spring-security-core/tree/7.0.x/buildSrc/src/main/groovy/grails/plugin/springsecurity