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

...now that 0.8 is also listed in the Reactor bom.

Issue: SPR-16387

Issue: SPR-16387

Issue: SPR-16387

Issue: SPR-16470

Includes upgrade to Hibernate ORM 5.2.17 for integration tests.

Issue: SPR-16780

Gradle 4.6 provides built-in support for the JUnit Platform within the
standard `test` task.

This commit configures a custom `testJUnitJupiter` test task for
executing JUnit Jupiter tests directly on the JUnit Platform instead of
indirectly on JUnit 4 via @RunWith(JUnitPlatform.class).

This switch provides for better integration with Gradle's test reporting
and paves the way for a possible transition to the JUnit Platform in the
future.

Issue: SPR-16672

Includes upgrade to Groovy 2.4.15 and HtmlUnit 2.30.

Issue: SPR-16390

Includes H2 1.4.197 and Selenium HtmlUnit Driver 2.29.3 as well.

The apiguardian-api dependency is once again a mandatory transitive
dependency of JUnit 5 since JUnit Jupiter 5.0.3.

Issue: SPR-13574

Includes upgrade to Hibernate Validator 6.0.4 (where applicable)

Issue: SPR-16115