Change pipeline to only be triggered automatically on ready merge request

Currently we are triggering the pipeline on every push. This is creating a lot of extra workload for our runners while the vast majority of these pipelines are most likely not used. In particular, after a merge we run the EXACT same pipeline on main again. To solve this, we should change when we trigger pipeline:

• Trigger automatically only on merge request event
• If the MR is a draft, it should be possible to trigger the pipeline, but devs will have to do so manually
• No pipeline on main after a merge

We of course leave all scheduled and externally triggered pipelines alone.