fix(GlButton): Fix button text color in dark mode

What does this MR do?

Use the rule added in !2778 (merged) to not invert button text color in dark mode.

gl-text-white is #333 in dark mode (that is not ideal but it is the situation for now..). So instead we use this value intended as light text contrast color.

before	after

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Accessibility

• Color combinations have sufficient contrast.