Fallback for colour scales

[teunbrand]

This issue was identified in #6287.

Some packages use scale_colour_continuous(low = "blue", high = "red"). This is only valid when end-users haven't touched the default "ggplot2.continuous.colour" options. The pattern these packages use are thus fragile and instead they should use scale_colour_gradient().
Likewise for scale_colour_discrete() and scale_colour_hue() and the fill aesthetic equivalents.

However, because the problem is quite ubiquitous (39 packages), we should have fallbacks in place.
I'll suggest that we use a grumpy fallback that always throws a warning, as the usage remains incorrect.