Java 21 follow-up cleanup

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 #`

## Trivial

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

## Not Trivial/tracked elsewhere

- [ ] #5827

Start by locating eligible Duration positive checks and ExecutorService usages in the repository; no specific files or tests are named. Complete the applicable mechanical cleanups, use try-with-resources where appropriate, and link any broader or already tracked work rather than expanding this umbrella issue.

Written by the indexing model from the issue text.