`theme_classic()` panel grid should be set blank at parent

[teunbrand]

This was brought to my attention by @EvaMaeRey. Currently, we have the following situation where we cannot easily switch on the grid in theme_classic(). In the plot below, we would expect green gridlines.

library(ggplot2)

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme_classic()

p + theme(panel.grid = element_line(colour = "limegreen"))

^{Created on 2025-02-05 with reprex v2.1.1}

This is because theem_classic() sets the child elements to blank:

ggplot2/R/theme-defaults.R

Lines 499 to 500 in b87a6e3

	panel.grid.major = element_blank(),
	panel.grid.minor = element_blank(),

Whereas it would make more sense to set the parent panel.grid to blank and let child elements inherit the usual route.