Pass missing arguments in guide_coloursteps()

[teunbrand]

This PR aims to fix #5930.

Briefly, I forgot to pass position and available_aes to new_guide(), which this PR fixes.

Reprex from issue:

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

ggplot(mpg, aes(displ, hwy, colour = cty)) +
  geom_point() +
  guides(color=guide_colorsteps(position = "bottom"))

^{Created on 2024-06-04 with reprex v2.1.0}

[thomasp85]

LGTM once a NEWS bullet is added