fix(Buttons): Remove px-to-rem for some effects and use variables

What does this MR do?

• Removes the px-to-rem() function from values that should remain as pixels, like borders and shadows, etc.
• Replaces pixel values with $gl- variables where applicable.

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.

Closes #892 (closed)