fix: URL to GitLab CSS bundle

What does this MR do?

Changes the URL to the GitLab CSS bundle to make Include GitLab CSS bundle working again.

Right now enabling this checkbox throws an error: Refused to apply style from 'https://gitlab-org.gitlab.io/gitlab/gitlab.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled. This happens because the target URL returns 404.

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.