Cleanup Capella Jenkins instance

Summary

While preparing the migration of the Capella Jenkins instance to our new backend storage device, I noticed the following:

several jobs/branches that did not run in the last 2-6 years
several Multibranch jobs that have a high number of branches, with many of them also being old and outdated
considerable disk space usage (58 GB)

Some examples:

https://ci.eclipse.org/capella/job/capella-addon-vpms-v1.2.x/
- Last successful build was more than 6 years ago
https://ci.eclipse.org/capella/job/capella-product/
- Total disk usage: 11GB
- 31 branches, with several branches that did not run for more than 2 years
- 21 PRs, several of them older than 1 year
- several builds are marked as "keep forever"
https://ci.eclipse.org/capella/job/capella-addon-textual-editor/
- Total disk usage: 11GB
- 19 branches, 17 pull requests (several of them outdated)

Please merge https://github.com/eclipse-capella/capella-xhtml-docgen/pull/282 and https://github.com/eclipse-capella/capella-textual-editor/pull/135 to reduce the number of builds kept with build artifacts
Please consider all build jobs during clean up and decide if they are still needed.

Please keep in mind that Jenkins is not a file server to archive build artifacts. Any build artifacts that should be kept around for some time, should be copied to the project's download directory at download.eclipse.org.