Use `Jenkins.get()` instead of deprecated method and minor improvements

[StefanSpieker]

Use Jenkins.get() instead of deprecated method and minor improvements:

• Use StandardCharsets.UTF_8
• removed unused imports

Testing done

All tests passed. Jenkins came up normally in debug environment.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue