[VSA parity] Support value stream and stage creation for ProjectNamespace

Currently, the GroupValueStream and GroupStage models can only be associated with a Group. To support ProjectNamespace we need to change the association definition.

Example (see the association changes in the model files): !101361 (closed)

Additionally:

• Add validation to the models to only accept Group and ProjectNamespace instances for the associations.
• Rename the GroupValueStream and GroupStage models (remove the Group prefix).
• The table name (group_value_streams) and the FK column (group_id) name can stay as is.
  • Maybe at some point, we will migrate the data to new tables. (could be a tech debt task)
• In the related services (ValueStreams::CreateService), rename the group parameter to namespace (or parent)