[App Arch] migrate legacy CSS to new platform

[lukeelmers]

Blocked by #53532

There are a number of places where we have left behind some CSS in the legacy world due to lack of support for it in the new platform, and as a workaround are importing that CSS into the new platform. This should now be unblocked, so we need to move over any legacy CSS as a cleanup step.

@streamich has started this work in #52414, so we can either update/merge that PR, or copy the work he has done so far into a new PR and close the original.

Here's a list of app arch legacy items I've found so far which contain some scss (be sure to look around for anything I may have missed):

• core_plugins/data
• core_plugins/embeddable_api
• core_plugins/dashboard_embeddable_container
• core_plugins/inspector_views
• core_plugins/interpreter
• [ ] core_plugins/kibana app team will handle this
• core_plugins/kibana_react (+ markdown components) [[App Arch] migrate legacy CSS to new platform (core_plugins/kibana_react) #59882] @VladLasitsa
• core_plugins/visualizations [move visualizations plugin to new platform #60403]
• ui/courier
• [ ] ui/error_auto_create_index mainly used by saved objects. leave behind for now.
• [ ] `ui/field_editor leave behind for now. will move with the index patterns ui.
• ui/share (? might be handled by app team) [[App Arch] migrate legacy CSS to new platform (ui/share) #59981] @VladLasitsa
• [ ] ui/vis will get moved with visualizations plugin
• [ ] ui/visualize will get moved with visualizations plugin
• any references in the root ui/_index.scss

Related to #58369

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[streamich]

I have a PR open that addresses this for some plugins: #52414

[joshdover]

Should be unblocked on this now that we have CSS and SCSS imports in Webpack supported for New Platform plugins.

[lukeelmers]

I think this can be closed. ui/_index.scss is currently only referencing the management plugin, which folks can import directly as they migrate.