fix(GlButton): render label buttons as spans

What does this MR do?

This changes the label button variant so that it renders a span instead of a button to improve accessibility.

To verify that this doesn't introduce any visual regression, the initial commit added a label button story, screenshots were then generated for that new story, and the actual change to render a span was then implemented.

Screenshots

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: gitlab!47695 (closed)
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Closes #1010 (closed)