Update available cmdstan version for conda

[WardBrian]

Submission Checklist

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

Summary

Since conda-forge/cmdstan-feedstock#9 was merged, the oldest version of cmdstan available on Conda is actually 2.26.1 not 2.27.0. It will probably never go further back in time though

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

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

[WardBrian]

I'm also using this PR to delete some unused CI files from the top level of the repo

[codecov-commenter]

Codecov Report

Merging #477 (05e9b94) into develop (21fb101) will decrease coverage by 0.48%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #477      +/-   ##
===========================================
- Coverage    78.03%   77.54%   -0.49%     
===========================================
  Files           30       10      -20     
  Lines         8991     2997    -5994     
===========================================
- Hits          7016     2324    -4692     
+ Misses        1975      673    -1302     

Impacted Files	Coverage Δ
cmdstanpy/cmdstanpy/model.py	84.06% <0.00%> (-0.78%)	⬇️
...ner/work/cmdstanpy/cmdstanpy/cmdstanpy/progress.py
...ner/work/cmdstanpy/cmdstanpy/cmdstanpy/_version.py
a/cmdstanpy/cmdstanpy/cmdstanpy/cmdstan_args.py
a/cmdstanpy/cmdstanpy/cmdstanpy/progress.py
...ork/cmdstanpy/cmdstanpy/cmdstanpy/compiler_opts.py
...k/cmdstanpy/cmdstanpy/cmdstanpy/install_cmdstan.py
a/cmdstanpy/cmdstanpy/cmdstanpy/stanfit.py
...runner/work/cmdstanpy/cmdstanpy/cmdstanpy/model.py
a/cmdstanpy/cmdstanpy/cmdstanpy/install_cmdstan.py
... and 12 more

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[mitzimorris]

LGTM!