CI jobs fail to pull data from the dagshub repository

[seisman]

Description of the problem

Our CI jobs start to fail to pull data from the dagshub repository recently. It's unclear what's happening but the dagshub website suggests downgrading dvc version to 2.3.0.

I'll open a PR to pin the dvc version to 2.3.0, but we need to unpin it after the issue is fixed.

[weiji14]

For reference, the bug was reported at iterative/dvc#6599 and fixed in dvc 2.8.0 released 12 Oct 2021. So things should work now.