Fixup OpenAPI content in Whats New doc

[mikekistler]

This PR makes some fixes to the OpenAPI content in the Whats New in ASP.NET Core 9.0 doc.

There was some content that didn't need to be preserved because it described changes from an earlier preview, and there was some duplicated content that I removed. The rest are very minor fixes.

---

Internal previews

📄 File	🔗 Preview link
aspnetcore/release-notes/aspnetcore-9.0.md	aspnetcore/release-notes/aspnetcore-9.0

[captainsafia]

I see the sections:

• Support for schema transformers on OpenAPI documents
• Support for [Required] and [DefaultValue] attributes on parameters and properties

in the preview that should be removed too although it is not clear to me what Markdown they are being sourced from.

[mikekistler]

@captainsafia The two other sections you mentioned were also in the completions .md, which I removed, so they should be gone now (I checked and they do appear to be done).