Migrate gitlab-ui into design.gitlab.com repository

• @markrian will implement this phase.
• Subsequent phases depend on this.

In order to minimise disruption to development, the transition from gitlab-org/gitlab-ui> to a monorepo needs to happen quickly. We don't want to block changes on gitlab-org/gitlab-ui> for long, and we also don't want to have to sync changes from gitlab-org/gitlab-ui> to gitlab-org/gitlab-services/design.gitlab.com> repeatedly. Here's how I see this working:

• Prepare a script and/or series of patches that performs the merging of the repositories. How this script/series of patches works will be informed by Phase 1.
• Once confidence in the approach is high (i.e., after reviews and test runs of it are acceptable), announce widely a "merge day", during which:
  1. The gitlab-org/gitlab-ui> repository will be locked and marked for archival. This means no new merge requests, issues, or repo pushes.
  2. The script/series of patches will be applied to merge gitlab-org/gitlab-ui> into gitlab-org/gitlab-services/design.gitlab.com>
• The announcement would also include information about changes to workflows, including:
  • gitlab-org/gitlab-services/design.gitlab.com> being the place to go for everything to do with @gitab/ui, meaning issues, merge requests, documentation, features, etc.
  • Semantic commits being replaced with changesets

Once that's done, we'll still have working @gitlab/ui releases being created by the gitlab-org/gitlab-services/design.gitlab.com> repository.

Post-merge things to address

• Post a comment in open GitLab UI merge requests with migration information, and close the merge requests.
• Update gitlab-org/gitlab-ui>'s default merge request and issue templates to point to this project instead.
• Ensure all labels defined in gitlab-org/gitlab-ui> are defined in gitlab-org/gitlab-services/design.gitlab.com>
• Move open issues to gitlab-org/gitlab-services/design.gitlab.com>
• Update gitlab-org/gitlab-ui>'s Pages site to redirect to new location, or add landing page pointing to new location, or add common docs page block that points to the new location.
• Add documentation about using changesets/publishing releases. !4502 (merged)
• Updated CODEOWNERS. Perhaps just need to set @gitlab-org/maintainers/gitlab-ui? !4504 (merged)
• Ensure all members of gitlab-org/gitlab-ui> are members of gitlab-org/gitlab-services/design.gitlab.com>, with the same roles? Perhaps just need to invite @gitlab-org/maintainers/gitlab-ui?
• Fix integration branch creation (https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/jobs/10202019937)
• Create Releases when publishing npm packages, so that Renovate updates of @gitlab/ui include the changelog. !4522 (merged) Edit: it turns out this wasn't enough.
• More exploration into getting changelogs to show up in Renovate MRs. gitlab-org/frontend/renovate-gitlab-bot!1176 (merged)
• Fix ui:update_screenshots job !4524 (merged)
• Add publish dry run job !4529 (merged)
• Update Renovate bot configuration (undo disabling added in gitlab-org/frontend/renovate-gitlab-bot!1153 (merged)) gitlab-org/frontend/renovate-gitlab-bot!1172 (merged)
• Use proper workspaces !4493 (merged)
• Update design.gitlab.com's Storybook iframes to point to new location. (This is already done, but the default location should probably be updated.) !4501 (merged)
• Update reviewer suggestions in Dangerfile !4519 (comment 2539219844) !4560 (merged)
• Update references to gitlab-org/gitlab-ui> in gitlab-org/gitlab-services/design.gitlab.com> to point to package subdirectory instead. !4530 (merged)
• Hoist ESlint (see !4528 (comment 2546048862)) !4718 (merged)
• Hoist Prettier (see !4528 (comment 2546048862)) !4715 (merged)
• Hoist markdown-lint !4728 (merged)
• Hoist yarn-deduplicate !4729 (merged)
• Hoist lefthook !4731 (merged)
• Hoist stylelint !4731 (merged)
• Fix the GDK's gitlab_ui.enabled configuration gitlab-org/gitlab-development-kit!4911 (merged)
• Update team.yml projects definitions (e.g., migrate gitlab-ui: maintainer frontend to design.gitlab.com: maintainer frontend). Not necessary, it's fine as-is.
• Update Reviewer Roulette: remove gitlab-ui project, and check that reviewers are valid. Not necessary, it's fine as-is.