Remove dependency management noise from POMs

Prior to this commit, the generated POMs for Spring Framework modules
would contain unneeded/harmful information from the Spring Framework
build:

1. The BOM imports applied to each module by the dependency
management plugin, for example for Netty or Reactor Netty.
Spring should not export that opinion to its POMs.

2. The exclusion of "org.slf4:jcl-over-slf4j" from *all* dependencies,
which made the POMs much larger than necessary and suggested to
developers that they should exclude it as well when using all those
listed dependencies. In fact, only Apache Tiles currently brings that
transitively.

This commit removes that information from the POMs.
The dependencyManagement Gradle plugin is disabled for POM generation
and we manually resolve the dependency versions during the generation
phase.
The Gradle build is streamlined to exclude "org.slf4:jcl-over-slf4j"
only when necessary.

Issue: SPR-16893

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

@@ -1,11 +1,5 @@
 description = "Spring Framework (Bill of Materials)"
 
-dependencyManagement {
-	generatedPomCustomization {
-		enabled = false
-	}
-}
-
 configurations.archives.artifacts.clear()
 artifacts {
 	// work around GRADLE-2406 by attaching text artifact