Static Site Editor: Migrate isSupportedContent to Apollo

What does this MR do?

Migrates the business logic to display the unsupported content view from Vuex to VueApollo. This MR is part of the effort to adopt GraphQL in the Static Site Editor. You can see more details of the frontend efforts at #216186 (closed).

To understand the whole effort, see &3183 (closed)

Next Merge Request (loading source content): !31268 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content