Replace superseded coord_flip() with swapped aesthetics in ppc_error_scatter_avg

[ishaan-arora-1]

ppc_error_scatter_avg() and the deprecated ppc_error_scatter_avg_vs_x() in R/ppc-errors.R use coord_flip(), which is marked as [Superseded] in ggplot2.

From the ggplot2 docs:

This function is superseded because in many cases, coord_flip() can easily be replaced by swapping the x and y aesthetics, or optionally setting the orientation argument in geom and stat layers.

https://ggplot2.tidyverse.org/reference/coord_flip.html

Two call sites at lines 260 and 330 of R/ppc-errors.R.

Would it be welcome to replace these with swapped aesthetics? Happy to open a PR if so.