Use new endpoint for POST when saving attachments

[ivarne]

Description

In Altinn/app-lib-dotnet#850, we add a new endpoint for POST, so that service owners can write code that modifies instance data (mutate form data or add/remove elements) when attachments gets uploaded.

Frontend needs to use the new endpoint and update its internal state when the backend version is new enough (likely 8.5.0 (not preview), we will see)

Additional Information

No response

[olemartinorg]

Assigning myself for testing.