feat(css): Add border-t-transparent utility mixin

What does this MR do?

This MR adds gl-border-t-transparent utility mixin, to be able to remove the temporary class added in gitlab!39705 (merged)

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
  • Customers Portal: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.