Migration to new jenkins

[dylex]

Migration to jenkins-new, significant cleanup of Jenkinsfile, mostly the same behaviors.

• Replace downstream_ branches (on stan) with downstream parameter
• Replace use of stanorg/ci:gpu-cpp17 image with local Dockerfile, mainly an optimized image with a few unnecessary things removed and updates to build correctly

[WardBrian]

Overall looks great, thanks!

I didn't see a run that actually built the distribution tests, so I've kicked off https://jenkins-new.flatironinstitute.org/job/CCM/job/Stan/job/math/job/jenkins-new/16/ to test

I'd also like to perhaps move the docker images back to one centralized place, especially since they would ideally also be used by stanc3: https://github.com/stan-dev/stanc3/blob/90c653249048b3aaa04bd488fcf20dceebeeda62/Jenkinsfile#L166
But that can be done post-merge

[dylex]

One complication to keep in mind for the Dockerfiles is that there are (at least) two different versions being used: stanorg/ci:gpu, which is based on an old version of Dockerfile-gpu before clang was updated to 7 (possibly https://github.com/stan-dev/ci-scripts/blob/b57fa182797941f87a92d509da9121952ac60cc0/docker/ci/Dockerfile-gpu) and stanorg/ci:gpu-cpp17 which is at least closer to the current version. Only math uses the new version as far as I can tell, and the others use the old one.

It's definitely possible to use a Dockerfile from a submodule or different repo, if we want.

[WardBrian]

@dylex understood -- I think it would be fine (probably even preferrable) if everything used the same image that Math does

[WardBrian]

(I'd still like to see a run that does all the distribution tests before merging, if one exists)