Deprecate palette/opacity.css and foundations/opacity.css

[pseys]

In order to support Alpha Button #4280 alpha values for black and white need to be simplified and ideally standardised across Salt and Salt Next. The Salt Next structure works across both color palettes.

The following .css files can be replaced and deprecated:

• palette/opacity.css
• foundations/opacity.css
• foundations/fade.css

Foundations/alpha.css is required to replace fade.css but requires fewer tokens and is consistent with SaltNext

[mark-tate]

Tecno Goal: in-progress
What is the end result?
Once we know, we can apply to code.
Paul/Josh to sync up

[mark-tate]

currently focused on these token changes, before we complete spacing token changes.

[mark-tate]

both complete by EOS, requires final review