VSA persisted default stage cannot be removed or edited

Bug: A persisted value stream cannot be edited when the name is 'default'.

Value stream analytics provides an in-memory value stream called 'default' which was possible to edit and persist in the past. It's also possible to create a new value stream called 'default' which would replace the in-memory value stream.

How to reproduce it:

1. Make sure to have at least a premium license.
2. Create a group.
3. Go to Analytics > Value Stream
4. Notice that we loaded the default value stream (in-memory).
5. Create a new value stream (top right dropdown). Call the value stream 'default'.
6. Now this value stream cannot be edited.

Expected: persisted value streams should be editable.