Removing support for non-layoutset apps

[olemartinorg]

Description

We should remove support for non-layoutset apps. This would require that Studio has a tool for converting existing apps to use layout-sets, and that our app-template also uses layout-sets.

Removing this gets rid of a known source of bugs, and makes it more obvious where one should put configuration for new functionality (such as the custom receipt, see #1468).

Additional Information

We also have to implement:

• Layout-set as default in app template app-template-dotnet#159

Bugs caused by not taking non-layoutset apps into consideration:

• Fix/no layout sets #1121
• Fix: Nullcheck layoutSets #438

[olemartinorg]

We should have support for converting apps in the upgrade tool as well. 🤩 And we have to double-check the tool that exists in Studio before we can consider this.

[RonnyB71]

cc: @nkylstad are you ready for this? We will create a migration for this in the conversion tool and update the template as well.