Let Tekton manage concurrent PipelineRuns automatically

[michaelsauter]

We have implemented a basic concurrency mechanism but this kind of functionality should better be provided by Tekton directly. This issue exists to track any progress that is happening in Tekton to see if and when we can make use of Tekton functionality instead of our own.

• TEP ~ Automatically manage concurrent PipelineRuns tektoncd/community#716
• https://hackmd.io/GK_1_6DWTvSiVHBL6umqDA