Please install the git-forensics plugin.
In release 8.5.0 of the warnings plugin I extracted the code to compute the reference build (the build that will be used as baseline for warnings delta computation) into the separate git-forensics plugin. While the old way still is supported I want to remove that part of the warnings plugin in the near future. So it would make sense to use and test the new plugin in our CI infrastructure. I will prepare a followup PR for our pipeline library as well so that all builds will use the new steps automatically.