Jenkinsfile: Rewrite from scratch.

[UlrichEckhardt]

In relation to #425 or, rather, the change in direction to use ci.travis.io infrastructure instead of GitHub Actions. Since the Jenkinsfile seems to have been unused for quite some time (last commit 2017), I thought one approach was to start from scratch to get it working first and then see which parts can be salvaged. Also, jenkins-infra/helpdesk#2776 comments indicate that the current file is not applicable to the target service.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• 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

[timja]

Non maintainers can't really make CI changes here without testing in a local setup as their changes to the CI configuration will be ignored for security

[UlrichEckhardt]

Yes, whatever change I made wasn't picked up, so I can't help here. Maybe the approach of rewriting from scratch is a useful approach though.

[nre-ableton]

Thanks for the effort nonetheless @UlrichEckhardt!