Follow recommended practice for using ggplot2::aes() in packages

[jdblischak]

I updated the use of ggplot2::aes() to follow the best practice recommendations from the ggplot2 maintainers in their vignette Using ggplot2 in packages:

If you know the mapping or facet specification is col in advance, use aes(.data$col) or vars(.data$col).

You will see a lot of other ways to do this in the wild, but the syntax we use here is the only one we can guarantee will work in the future!

Trying to find the minimum required version of {ggplot2} to support the current best practice required some trial and error. Support for tidy evaluation was first added in version 3.0.0 in July 2019 (CRAN archives). However, the test suite failed with this version. By binary searching through the potential versions, I determined that version 3.1.1 is the minimum required version. It's a patch release from April 2019 that for some reason doesn't have its own NEWS entry. The test suite passes with the version, with the minor exception that a few of the snapshot tests fail because the order of the legends is reversed (ie only the display, and not the actual content, was affected).