maven.gitlab-ci.yml: review rules configuration for all jobs

The rules configuration in the maven.gitlab-ci-yml template could do with a quick review. We need to make sure that they follow the standards required by the common-pipeline.yml template.

Acceptance criteria

• All jobs are able to be disabled by a *_DISABLED variable
• The jobs should be enabled/gated by the existence of a config file (pom.xml in this case I believe)
• We should not disable jobs for merge request pipelines in this template.
  • The common pipeline uses the workflow-branch-pipelines.gitlab-ci.yml template to do this instead. This gives greater flexibility for edge cases which may want to use merge request pipelines for specific reasons.