Silence new density() warning

[teunbrand]

This PR aims to prevent a newly introduced warning.

From the R4.3.0 NEWS bullets:

density(x, weights = *) now warns if automatic bandwidth selection happens without using weights; new optional warnWbw may suppress the warning. Prompted by Christoph Dalitz' PR#18490 and its discussants.

This affects ggplot2 when it uses weights:

library(ggplot2)

cutoff <- min(mtcars$mpg)

p <- ggplot(mtcars, aes(mpg, weight = cyl)) +
  stat_density()
invisible(ggplotGrob(p))
#> Warning in density.default(x, weights = w, bw = bw, adjust = adjust, kernel =
#> kernel, : Selecting bandwidth *not* using 'weights'

^{Created on 2023-04-22 with reprex v2.0.2}

Because of backwards compatibility with older R versions, we cannot use the warnWbw argument. This PR prevents this warning by precomputing the bw argument of density() and noting in the docs that weights are ignored for bandwidth calculation (as they always have been, it is just stated now).

[thomasp85]

LGTM