Remove support for CmdStan pre-2.34 in a future version

[WardBrian]

Summary:

It would be nice to remove a lot of legacy code in the package which has been made obsolete by recent CmdStan updates. For example, supporting running multiple chains without using the num_chains argument to CmdStan

Current Version:

1.1

[WardBrian]

I updated my wish in the title to be only supporting 2.34 and newer so we can assume the existence of the save_cmdstan_config (#714) and save_metric (#713) arguments

After another year, it would make sense to make this 2.37, which will allow passing comma-separated lists of filenames and make a lot of our logic in runset completely obsolete