Include link button in gl-button styles

What does this MR do?

Currently, the .btn-link button isn’t paired with the .gl-button class, like other buttons are, and styles from the Bootstrap npm package appear to be impacting the appearance of the button, especially when paired with the .btn-secondary class that can be added from the knobs in Storybook.

State	Before	After
rest
:hover
:focus
:active:focus

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