chore: update story-viewer paths for dropdowns

What does this MR do?

Update <story-viewer> component paths for GlDisclosureDropdown and GlCollapsibleListbox to compliment change in gitlab-org/gitlab-ui!3731 (merged)

Steps to validate locally

• In gitlab-ui run branch move-new-dropdowns-stories locally (from: gitlab-org/gitlab-ui!3731 (merged))
• In design.gitlab.com update GITLAB_UI_URL in nuxt.config.js to local gitlab-ui URL e.g. http://localhost:9001

Does this MR meet the acceptance criteria?

• The MR title and commit messages meet the Pajamas commit conventions.
• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.

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