Java 21 follow-up cleanup

[DomGarguilo]

This ticket might be useful as an umbrella for Java 21 follow-up work since that is the current next target version.

One possible approach would be to track small mechanical cleanups here directly, while linking out to separate issues for anything broader, riskier, or already being tracked elsewhere. Feel free to edit this to add any new things that seem worthwhile.

For smaller PRs, we could use:
Partially addresses #<ticket number>

Trivial

• Replace eligible Duration positive checks with Duration.isPositive()
• Assess and move ExecutorService usages into try-with-resources

Not Trivial/tracked elsewhere

• Use LinkedTransferQueue poll w/ timeout after upgrade to JDK 21+ #5827