Fix colorbar of reversescale colorbars of heatmap-coloring contours

[etpinard]

fixes#4430

before: https://codepen.io/etpinard/pen/ZEYePxW
after: https://codepen.io/etpinard/pen/JjoWzYb

cc @archmoj

[archmoj]

Great! Could you please add a test for the case of using coloraxis as well?

[etpinard]

Ok I'll add a subplot on the new mock.

But in my opinion, we don't need that as the trace-colorscale vs coloraxis logic is inside Colorscale.extractOpts which is already tested elsewhere.

[archmoj]

If so, just testing in a codepen is enough.
Thank you!

[archmoj]

Concerning before codepen it is better to use v.1.51.3 not the latest.

[etpinard]

Concerning before codepen it is better to use v.1.51.3 not the latest.

Sure, edited above.

Great! Could you please add a test for the case of using coloraxis as well?

done in af025a2

[archmoj]

💃 💃 after all green light on the CI.