Refactor Gradle tasks in Spring Framework build

This commit reorganizes tasks and scripts in the build to only apply
them where they're needed. We're considering here 3 "types" of projects
in our build:
* the root project, handling documentation, publishing, etc
* framework modules (a project that's published as a spring artifact)
* internal modules, such as the BOM, our coroutines support and our
integration-tests

With this change, we're strealining the project configuration for all
spring modules and only applying plugins when needed (typically our
kotlin support).

See gh-23282

spring-framework-bom/spring-framework-bom.gradle

@@ -1,5 +1,7 @@
 description = "Spring Framework (Bill of Materials)"
 
+apply plugin: "maven"
+
 configurations.archives.artifacts.clear()
 artifacts {
 	// work around GRADLE-2406 by attaching text artifact