Review Jenkins Java Test Jobs / Configurations

The Java test jobs in Jenkins have been growing organically over time and need to be reviewed to ensure they're balancing appropriate coverage, using reasonable resources (e.g. infra costs, time), and are following a sane testing strategy:

• naming is uniform and predictable (especially for PR phrases)
• postcommit monolith is broken down into individual jobs to improve balancing across jenkins nodes
• old jobs are removed from Jenkins (e.g. [1, 2])
• README.md file is updated

[1]: https://ci-beam.apache.org/job/beam_PostCommit_Java11_Examples_Dataflow_Portability/

[2]: https://ci-beam.apache.org/job/beam_PostCommit_Java11_ValidatesRunner_PortabilityApi_Dataflow/