Swap size -> linewidth

At least in the scRNA-seq-advanced differential expression notebook (and maybe in more spots), we still have the ggplot2 size parameter. Needs to be linewidth these days to avoid warnings.

Locate the scRNA-seq-advanced differential expression notebook and search it, plus other notebooks, for ggplot2 uses of the size parameter. Replace the applicable uses with linewidth, then render or run the affected notebook(s) and confirm the deprecation warnings are gone.

Written by the indexing model from the issue text.