[colors] Deprecate yellow-20 and keep yellow-30 with v2 color palette update

[laurenmrice]

Need to deprecate yellow-20 and keep yellow-30 for v2 color palette.

Right now we have two yellow colors in elements. Yellow-20 is from the old color palette. Yellow-30 is the new color that will replace it.

[AlexanderLyon]

@laurenmrice is this across all components? It looks like the only place yellow-20 is currently being used is in a mix to create the warning notification background color

[laurenmrice]

Yes, the yellow-30 should be used in that mixin .