Add argument for minimal group size in boxplot

[teunbrand]

This PR aims to fix #6776.

Instead of mirroring dicook@7dbc03e, the threshold for what constitutes a small group is set via an argument. This preserves the current behaviour, while allowing for the requested behaviour. I'm still open for better argument names than min.group.n.

Note in example below, the first two groups are rendered as points without box and whiskers.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
df <- data.frame(
  x = rep(LETTERS[1:4], c(1, 5, 10, 50)),
  y = rnorm(66)
)

ggplot(df, aes(x, y)) +
  geom_boxplot(min.group.n = 6)

^{Created on 2025-12-19 with reprex v2.1.1}