Bump markdownz from 8.4.1 to 9.0.0 (#6902)

Bumps [markdownz](https://github.com/zooniverse/markdownz) from 8.4.1 to 9.0.0.
- [Release notes](https://github.com/zooniverse/markdownz/releases)
- [Changelog](https://github.com/zooniverse/markdownz/blob/master/CHANGELOG.md)
- [Commits](zooniverse/markdownz@v8.4.1...v9.0.0)

---
updated-dependencies:
- dependency-name: markdownz
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

app/components/ExternalLinksBlock/components/ExternalLink/ExternalLink.spec.jsx

@@ -34,11 +34,11 @@ describe('ExternalLink', function() {
         />
       );
       markdownLink = wrapper.find('span.link-title');
-      markdownHtml = markdownLink.props().dangerouslySetInnerHTML.__html;
+      markdownHtml = markdownLink.html();
     });
 
     it('should render the markdown hyperlink correctly', function () {
-      expect(markdownHtml).to.equal('<a rel="noopener nofollow noreferrer" target="_blank" href="https://www.google.com"></a>')
+      expect(markdownHtml).to.equal('<span class="markdown link-title"><a rel="noopener nofollow noreferrer" target="_blank" href="https://www.google.com"></a></span>')
     });
 
     it('should have the Font Awesome external link icon', function () {