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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