Consider removing occurrences of `aes_string()`

Currently, many of our plotting functions use `aes_string()` to handle data mappings. This function has been deprecated in newer `ggplot2` versions, so we may want to think about moving away from this strategy and instead using `aes()` with so-called "curly-curly"'s`{{}}`.
Although I am opening this issue in this repo, this code change might/is(?) something we should do elsewhere (`scpca-nf`?), since this repo is more internal. But, I didn't want to forget this so filing here now!

Locate all plotting functions that use aes_string() and review whether the related work belongs in this repository or scpca-nf. Confirm the scope before changing mappings to aes() with {{}}; done means the in-scope occurrences are migrated and the plotting behavior remains working.

Written by the indexing model from the issue text.