Fix warning regression in geom_dotplot()

[teunbrand]

This PR aims to fix a regression from #5267.

Briefly, geom_dotplot()/stat_bindot() started emitting warnings when these functions shouldn't. For example:

library(ggplot2) # current main branch

dat <- data_frame(x = rnorm(20), g = rep(LETTERS[1:2], 10))

ggplot(dat, aes(x, fill = g)) + 
  geom_dotplot(binwidth = .4, alpha = .4, binpositions = "all")
#> Warning: The following aesthetics were dropped during statistical transformation: bin,
#> bincenter
#> ℹ This can happen when ggplot fails to infer the correct grouping structure in
#>   the data.
#> ℹ Did you forget to specify a `group` aesthetic or to convert a numerical
#>   variable into a factor?

^{Created on 2023-04-19 with reprex v2.0.2}

This PR fixes that spurious warning by adding bin and bincenter to the dropped_aes field.

[thomasp85]

LGTM