Outdated minimum versions for suggested packages

[utkarshpawade]

Several Suggests packages in DESCRIPTION have minimum versions from 2016–2018:

• rstan >= 2.17.1 (2018)
• rstanarm >= 2.17.4 (2018)
• rstantools >= 1.5.0 (2018)
• knitr >= 1.16 (2017)
• rmarkdown >= 1.0.0 (2016)

Since bayesplot requires R >= 4.1.0 (May 2021), these floors are unnecessarily old and give users false confidence that ancient versions are supported. They should be bumped to versions that were current around the R 4.1.0 release.