Remove all `$theme-gray-{weight}` variables in favor of `$gray-{weight}`

What does this MR do?

Remove all $theme-gray-{weight} variables in favor of $gray-{weight}. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24268#note_130913811.

What are the relevant issue numbers?

Fix #56266 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer