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

Upgrade to JAXB 2.3, JAX-WS 2.3, Annotations 1.3.1, Interceptor 1.2.1.
Also includes Log4J 2.9.1 and Asciidoctor 1.5.6.

Latest dependency updates (POI 3.17, Rome 1.8, EhCache 3.4, Caffeine 2.5.6, RxJava 2.1.4, Tomcat 8.5.21, JRuby 9.1.13, Rhino 1.7.7.2)

Prior to this commit, various dependencies in the spring-test module
pulled in commons-logging as a transitive dependency. Consequently,
the presence of commons-logging in the generated Eclipse classpath
overrode the intended use of the spring-jcl module causing JUL to be
used instead of log4j for tests executed within Eclipse, thereby
ignoring the configuration in src/test/resources/log4j2-test.xml.

This commit addresses this issue by excluding commons-logging in
spring-test.gradle for all Selenium dependencies.

Issue: SPR-15930

Includes latest dependency updates (Netty 4.1.15, Undertow 1.4.19)

Issue: SPR-15808

This reverts commit 3e2f6c84.

This commit applies the Dependency Management Plugin to modules that
require it; right now Spring Framework is importing BOMs for Netty and
Reactor dependencies only.

Instead of applying those BOMs to all modules, they're applied only
where they're needed.

Issue: SPR-15885

The main `build.gradle` file contains now only the common build
infrastructure; all module-specific build configurations have
been moved to their own build file.

Issue: SPR-15885