Blocks Build: remove calypso-color-schemes dependency and rely on color-studio instead

Follow-up from #16965 and #16733

We can aim to remove it again in the future, but when we do so:

- [ ] We'll need to update `postcss.config.js` so it can really pull both SASS variables and CSS custom properties from `@automattic/color-studio` without the need for direct imports.
- [ ] We'll need to remove existing direct imports.
- [ ] We'll need to update all SCSS in the blocks to use Studio colors instead of the Calypso Color Schemes aliases.

Start with postcss.config.js and trace how SASS variables and CSS custom properties are imported from @automattic/color-studio. Then locate the existing direct imports and SCSS using Calypso Color Schemes aliases. Done means the dependency and direct imports are removed and all block SCSS uses Studio colors without breaking the build.

Written by the indexing model from the issue text.