Update to .NET 9 and replace Swashbuckle/Swagger with ASP.NET Core OpenAPI generation

[jasonjoh]

Since .NET 9 brings OpenAPI doc generation natively to ASP.NET Core, we no longer need the Swashbuckle dependency. This does introduce a few changes to note.

• The new generation only supports JSON document format. YAML format is planned but unclear when it will land. See Support emitting OpenAPI documents in YAML format dotnet/aspnetcore#58516
• New generation does not support consuming info from XML comments, so I had to move some things into attributes. The biggest gap here is the loss of example values in the schemas.
• Removing Swashbuckle means no more SwaggerUI page.

On a positive note, the OpenAPI is generated at build time rather than at run time.