GitLab integration test for gitlab-org/gitlab-services/design.gitlab.com!5931

This is a long-lived integration branch for Design updates: integrate Aura design system vi... (gitlab-org/gitlab-services/design.gitlab.com!5931 - merged).

What

1. Remove application_chrome_nav_item.scss, move .with-mouse-over-flyout styles to super_sidebar.scss
2. Remove page_theme_background.scss styles and page-theme-background class from application_helper.rb (and spec)
3. Remove themes from super_sidebar.scss
4. Replace var(--body-background-color) with background-color: var(--gl-application-chrome-background-color)
5. Remove var(--body-background-color) definition in super_sidebar.scss
6. Remove --super-sidebar-accent-foreground-color and usage in .super-sidebar-context-header
7. Add @apply gl-shadow-sm; to .paneled-view in app/assets/stylesheets/layouts/panel.scss
8. Remove --super-sidebar-theme-accent-color CSS custom property and usage
9. Remove aura_tinted_neutral.scss temporary style overrides

Do not merge this!

Workflow

1. Fix pipeline failures in this merge request, if any are needed.
2. When ready, get gitlab-org/gitlab-services/design.gitlab.com!5931 (merged) merged.
3. Close this merge request.
4. Wait for Renovate to create a version bump merge request for @gitlab/ui. You will automatically be pinged on that merge request and assigned to it.
5. If you made any fixes in this merge request, cherry-pick them into Renovate's merge request.
6. Get that Renovate merge request reviewed and merged.

Commit messages

We should write clear commit messages so we can associate them correctly. For example:

B1: Fix super-sidebar theme colors to old neutrals

- Issue: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/work_items/3444
- MR: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/merge_requests/5902

Checks

• The full Jest suite has run (i.e., not just predictive jobs)
• The full Jest suite has passed
• The full RSpec suite has run (i.e., not just predictive jobs)
• The full RSpec suite has passed

After gitlab-org/gitlab-services/design.gitlab.com!5931 (merged) merges

• Close this merge request.
• Cherry-pick fixes from here to Renovate's merge request.