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-web/spring-web.gradle

@@ -1,5 +1,7 @@
 description = "Spring Web"
 
+apply plugin: "kotlin"
+
 dependencyManagement {
 	imports {
 		mavenBom "io.projectreactor:reactor-bom:${reactorVersion}"