Bump minimum versions for suggested packages

[utkarshpawade]

Fixes #496

Summary

• Bump rstan to >= 2.21.0, rstanarm to >= 2.21.1, rstantools to >= 2.1.0, knitr to >= 1.33, rmarkdown to >= 2.11
• Aligns Suggests version floors with the existing R >= 4.1.0 requirement

[jgabry]

Thanks!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (a30a706) to head (c76612f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #497   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files          35       35           
  Lines        5857     5857           
=======================================
  Hits         5779     5779           
  Misses         78       78           

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[jgabry]

Thinking about this a bit more, my view is that DESCRIPTION minimum versions should mean “the oldest version bayesplot actually requires,” not “a version that happened to be current when R 4.1.0 came out.” We need package versions that are compatible with R 4.1 and compatible with the code used in bayesplot. So I would wait to change these version numbers unless we know that there is a compatibility issue.

That said, there might be compatibility issues with these versions and if that's the case we can change them, but we'd want to know that. If you want to look into that you can update this PR with just the ones that are incompatible (if any). Otherwise we can close this.