Fix value stream resource to allow creation at same time as group/project

The current gitlab_value_stream_analytics resource does not allow a user to create a group/project at the same time as the value stream, meaning the config for the value stream cannot reference the path of a group/project that is getting created in the same apply. The ModifyPlan errors stating the project or group path must be defined. This fixes the bug by removing the check from ModifyPlan and instead using a validator for ExactlyOneOf.