Migrate markdown toolbar buttons to Pajamas::ButtonComponent

What does this MR do and why?

References #417801 (closed)

It migrates markdown toolbar buttons in Repository single file editor to Pajamas ButtonComponent

• Go to some project repo
• Open markdown file - file with md extension in a single file editor. Example url: http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/edit/main/CONTRIBUTING.md
• Try using the toolbar and make sure it is working

Screenshots or screen recordings

You can see that After icons are slightly lighter than before as they use the correct button icon color. Though I'd like to validate it with the UX team.

Before
After