Update Microsoft.OpenApi to preview28

[captainsafia]

This pull request primarily updates the Microsoft.OpenApi package versions and refactors code to simplify imports and improve type usage. The most significant changes include updating dependencies, replacing outdated Microsoft.OpenApi.Models types with the new Microsoft.OpenApi namespace, and modifying public API definitions accordingly.

Dependency Updates:

• Updated MicrosoftOpenApiVersion and MicrosoftOpenApiYamlReaderVersion from 2.0.0-preview.18 to 2.0.0-preview.28 in eng/Versions.props.

Refactoring Imports:

• Removed references to Microsoft.OpenApi.Models, Microsoft.OpenApi.Models.Interfaces, and Microsoft.OpenApi.Models.References in favor of the new Microsoft.OpenApi namespace across multiple files, including XmlCommentGenerator.Emitter.cs, TransformersBenchmark.cs, and various transformer files in the sample directory. [1] [2] [3] [4] [5] [6] [7]

Code Adjustments:

• Replaced the initialization of schema.Extensions from an empty array ([]) to a Dictionary<string, IOpenApiExtension> in TransformersBenchmark.cs. [1] [2]

Public API Updates:

• Updated public API definitions in PublicAPI.Shipped.txt to reflect the transition from Microsoft.OpenApi.Models types to Microsoft.OpenApi types. This includes changes to method signatures and transformer interfaces. [1] [2] [3]

Project Configuration:

• Added <Using Include="Microsoft.OpenApi" /> to the Sample.csproj and Microsoft.AspNetCore.OpenApi.csproj files to simplify namespace management. [1] [2]

[martincostello]

Breaking change announcement probably needed for API changing?

Comparing main vs this PR, there's already a tonne of breaking changes via Microsoft.OpenApi in already shipped previews of 10. This is just even more churn on top of what we've had already 😅.

[captainsafia]

/backport to release/10.0-preview6

[github-actions[bot]]

Started backporting to release/10.0-preview6: https://github.com/dotnet/aspnetcore/actions/runs/15749528712