Replace $gl-gray- variables with $gray- variables

What does this MR do?

• Deprecate $gl-gray- variables and replace instances with $gray- variables that are the closest hex value without shifting lighter.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Closes #231490 (closed)