Remove remaining deprecations, very old cmdstan version checks

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Removes the remaining deprecated code paths and closes #685. I took the somewhat-conservative latest-plus-two-versions path here, so I left checks for 2.35/2.36, but deleted any older than that.

This also clears the path for us to tackle #713 and #714

Copyright and Licensing

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[amas0]

This looks good.

I think this means we can also remove the test and testing csv mentioned in #805 ; which, if I recall, could only be produced by a cmdstan version we are no longer supporting.

[WardBrian]

The change to make #805 go away was made in 2.35 I believe, so it's just within the current-minus-two range.