Clean up PMD rules/warnings

[jwaeab]

The PMD rules/warnings haven't been maintained for a long time. A more thorough work should be done:

• Step to a newer PMD version
• Go through current rules/exceptions and make sure they are up to date
• Do some actual code cleaning according to the warnings

[0rlych1kk4]

Hi, I’d be interested in helping with this cleanup effort if it’s still open for contribution. Before starting, I wanted to clarify a few things to align with project expectations:

• Is there a target PMD version the project would like to upgrade to?
• Are there rules that are intentionally suppressed today that should remain suppressed?
• Would maintainers prefer incremental cleanup PRs (rule groups / modules) rather than a single large refactor?

I’m happy to begin by auditing current warnings and proposing a scoped cleanup plan.