Define merge request pipelines for other targets than main

Currently, the merge request pipeline has been tuned for merges to main. However, the pipeline is also created for any other merge request. With larger works in branches, it is very practical to use merge requests to collect content into the feature branch but the content might not always fulfil the demands of the current merge request pipeline. Thus, it would be good to define and implement the pipeline for merge requests that merge to other targets than the main branch. A couple of options would be following:

1. Reduce non-main merge request pipeline to run only a few tests that can be allowed to fail. Other jobs can be skipped (or not even run).
2. Run all the same tests but make everything allowed to fail. This can be with a merge request title tag to be clear that it is an intentional risk.
3. Skip the whole pipeline for non-main target merge requests.