Investigate if we need to add GITLAB_CDN_HOST in the `frame-src` and `worker-src` directives of the CSP

Follow-up to #336938 (closed)

GitLab.com has https://assets.gitlab-static.net in the frame-src and worker-src directives. That host actually isn't used anymore for assets and could be completely removed but it raises the question if GITLAB_CDN_HOST should be added to the frame-src and worker-src directives as we already do for script-src, style-src and font-src.