Pin distributed to 1.21.6
Pin distributed to 1.21.6

Appears that there are some issues shutting down Distributed Client's in
1.21.8, so pin to 1.21.6 so that we can finish the rebuild of the stack
for now. Can drop the pinning once the upstream issues are fixed.

DudLab/docker_nanshe_workflow,DudLab/docker_nanshe_workflow,nanshe-org/docker_nanshe_workflow,nanshe-org/docker_nanshe_workflow