Bump level of panel grid blanknness in theme_classic()

[teunbrand]

This PR aims to fix #6320.

Instead of setting panel.grid.major and panel.grid.minor to blank, we set the parent panel.grid to blank.

Reprex from issue:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading 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}

[thomasp85]

LGTM