Enable ColorVariable in scss-lint

What does this MR do?

• Enable the ColorVariable rule in.scss-lint.yml
• Change all color literals in app/assets/stylesheets to color variables

Why was this MR needed?

• Enforce the usage of color variables to better constrain the various color values being used

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23500 (closed)