Prior to this commit, the Spring Framework build would partially use the
dependency management plugin to import and enforce BOMs.

This commit applies the dependency management plugin to all Java
projects and regroups all version management declaration in the root
`build.gradle` file (versions and exclusions).

Some versions are overridden in specific modules for
backwards-compatibility reasons or extended support.

This commit also adds the Gradle versions plugin that checks for
dependency upgrades in artifact repositories and produces a report; you
can use the following:

    ./gradlew dependencyUpdates