Deprecate additional_course_content_plugin plugin slot

So it sounds like we can simplify this quite a bit!

If we remove this block:
https://github.com/openedx/frontend-app-authoring/blob/11de4022f0ca86f0991c6f4cdcf612497c7a24b8/src/pages-and-resources/PagesAndResources.jsx#L48-L58

and this line
https://github.com/openedx/frontend-app-authoring/blob/11de4022f0ca86f0991c6f4cdcf612497c7a24b8/src/pages-and-resources/PagesAndResources.jsx#L71

and this block
https://github.com/openedx/frontend-app-authoring/blob/11de4022f0ca86f0991c6f4cdcf612497c7a24b8/src/pages-and-resources/PagesAndResources.jsx#L96-L106

then everything should be in the one PageGrid.

We then need to handle the 2 slot id issue. By removing those and making no other changes we'd only be supporting:

• Site operators using only org.openedx.frontend.authoring.additional_course_plugin.v1/additional_course_plugin

And we need to continue to also support:

• Site operators using only org.openedx.frontend.authoring.additional_course_content_plugin.v1/additional_course_content_plugin
• Site operators using both org.openedx.frontend.authoring.additional_course_plugin.v1/additional_course_plugin and org.openedx.frontend.authoring.additional_course_content_plugin.v1/additional_course_content_plugin

I think the best way to handle this would be to update the AdditionalCoursePluginSlot component

https://github.com/openedx/frontend-app-authoring/blob/11de4022f0ca86f0991c6f4cdcf612497c7a24b8/src/plugin-slots/AdditionalCoursePluginSlot/index.tsx#L3-L8

to include the plugin slot from the AdditionalCourseContentPluginSlot component

https://github.com/openedx/frontend-app-authoring/blob/11de4022f0ca86f0991c6f4cdcf612497c7a24b8/src/plugin-slots/AdditionalCourseContentPluginSlot/index.tsx#L4-L7

so we'd have

export const AdditionalCoursePluginSlot = () => (
  <PluginSlot
    id="org.openedx.frontend.authoring.additional_course_plugin.v1"
    idAliases={['additional_course_plugin']}
  />
  <PluginSlot
    id="org.openedx.frontend.authoring.additional_course_content_plugin.v1"
    idAliases={['additional_course_content_plugin']}
  />
);

at that point we can completely remove the AdditionalCourseContentPluginSlot component.

We can then put out a DEPR for the org.openedx.frontend.authoring.additional_course_content_plugin.v1/additional_course_content_plugin id/alias and let site operators know they should move anything they're currently putting in that slot to the org.openedx.frontend.authoring.additional_course_plugin.v1 slot instead.

Originally posted by @brian-smith-tcril in https://github.com/openedx/frontend-app-authoring/issues/2321#issuecomment-3117947497

Start with src/pages-and-resources/PagesAndResources.jsx and inspect the linked blocks and line, then compare src/plugin-slots/AdditionalCoursePluginSlot/index.tsx with AdditionalCourseContentPluginSlot/index.tsx. Done means both slot IDs remain supported through AdditionalCoursePluginSlot, the separate content slot component is removed, and the deprecated slot is no longer rendered separately.