Remove duplicate SCSS variables for Pajamas colors

What does this MR do and why?

Simplify color usage to reduce SCSS color functions (lighten(), darken(), mix() etc.) and duplicate SCSS variables for Pajamas color values. This should enable future color usage to inherit design token generated CSS custom properties from GitLab UI, in an effort to simplify dark mode color definitions and usage. Current SCSS color function usage can be viewed on the GitLab Code Scanner

• Replace $white-normal with $gray-50 ad72e5b6
• Replace $almost-black with $gray-950 14fa23f5
• Replace $black-transparent with $t-gray-a-24 5dc41a95

How to set up and validate locally

1. Review SCSS variable changes

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #430055