Restore old guides in guides()

[teunbrand]

This PR aims to fix #6165.

Briefly, it rectifies the mistake that we should've been testing old S3 guides with the 'guide' class instead of modern ggproto guides with the 'Guide' class.

Reprex from issue:

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

guides(x = guide_axis())
#> <Guides[1] ggproto object>
#> 
#> x : <GuideAxis>

guides(x = ggprism::guide_prism_minor())
#> <Guides[1] ggproto object>
#> 
#> x : <guide>

^{Created on 2024-10-29 with reprex v2.1.1}