lib: apply minor dtls cleanups

[jasnell]

Remove the object seal on the exports.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73665/

[mcollina]

lgtm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (c9562dd) to head (f9c3ada).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63539      +/-   ##
==========================================
+ Coverage   90.30%   90.32%   +0.02%     
==========================================
  Files         730      730              
  Lines      234188   234152      -36     
  Branches    43919    43909      -10     
==========================================
+ Hits       211478   211500      +22     
+ Misses      14443    14372      -71     
- Partials     8267     8280      +13     

see 39 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.