Add back UnnecessaryFullyQualifiedName rule in pmd ruleset

[Akshat-Jain]

Description

In #17466, we had added exclusion for UnnecessaryFullyQualifiedName rule to avoid a large number of irrelevant changes to the PR on upgrading maven-pmd-plugin version.

This PR re-visits that and adds back the UnnecessaryFullyQualifiedName rule, fixing all violations of the rule across the project.

---

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[gianm]

IMO the old style was better. Reading through the diff the new code often feels less clear. So I think we should revert this, and consider requiring that static methods be qualified. TBH I thought they were already required to be qualified outside tests.