Prior to this commit, the build would use a custom task to create a BOM
and manually include/exclude/customize dependencies. It would also use
the "maven" plugin to customize the POM before publication.

This commit now uses a Gradle Java Platform for publishing the Spring
Framework BOM. We're also now using the "maven-publish" plugin to
prepare and customize publications.

This commit also tells the artifactory plugin (which is currently
applied only on the CI) not to publish internal modules.

See gh-23282