Prep: WN 10 Prev 2 OpenAPI.NET include (#34981)

Just the prep include file.  Will be followed by a PR for changes to the original to be reviewed.

Author: wadepickett

aspnetcore/release-notes/aspnetcore-10/includes/OpenApiNetV2Prev7.md

@@ -0,0 +1,6 @@
+### Upgrade to OpenAPI.NET v2.0.0-preview7
+
+The OpenAPI.NET library used in ASP.NET Core OpenAPI document generation has been upgraded to v2.0.0-preview7. This version includes a number of bug fixes and improvements and also introduces some breaking changes. The breaking changes should only impact users that use document, operation, or schema transformers. Breaking changes in this iteration include the following:
+
+- Entities within the OpenAPI document, like operations and parameters, are typed as interfaces. Concrete implementations exist for the inlined and referenced variants of an entity. For example, an `IOpenApiSchema` can be an inlined `OpenApiSchema` or an `OpenApiSchemaReference` that points to a schema defined elsewhere in the document.
+- The `Nullable` property has been removed from the `OpenApiSchema` type. To determine if a type is nullable, evaluate if the `OpenApiSchema.Type` property sets `JsonSchemaType.Null`.