[BEAM-4050] Use new DSL methods for generated POM metadata

[swegner]

POM generation APIs were improved in Gradle 4.8; this change takes
advantage of new DSL APIs to define generated POM metadata when
available. Some metadata (notably dependencies) still need to be
directly added to the XML.

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[swegner]

I did a manual diff of the before-and-after of this change; a couple of the sections get re-ordered in the POM but otherwise the content is identical.

./gradlew :beam-sdks-java-core:publishToMavenLocal -Ppublishing && meld \
    pom_diff/before.xml  \
    ~/.m2/repository/org/apache/beam/beam-sdks-java-core/2.6.0-SNAPSHOT/beam-sdks-java-core-2.6.0-SNAPSHOT.pom

R: @lukecwik

[lukecwik]

This is pretty sweet clean-up.