Fix Java nightly snapshot build.
Fix Java nightly snapshot build.

After upgrading to Gradle 6.6.1 (commit
1db383c58c214ec2e10edec72e7fc427c74ebb1c) the snapshot builds started
failing. This is because the maven plugin now publishes Maven 3
metadata, where previously it published Maven 2 metadata. From the
userguide [1]:

The Maven Plugin used to publish the highly outdated Maven 2 metadata format. This has
been changed and it will now publish Maven 3 metadata, just like the Maven Publish Plugin.
With the removal of Maven 2 support, the methods that configure unique snapshot behavior
have also been removed. Maven 3 only supports unique snapshots, so we decided to remove
them.

[1]: https://docs.gradle.org/current/userguide/userguide.pdf

apache/beam,chamikaramj/beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,chamikaramj/beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,apache/beam