Consider deprecating `Microsoft.Extensions.ApiDescription.Client` package

[captainsafia]

The Microsoft.Extensions.ApiDescription.Client package ships MSBuild logic that is designed to support OpenAPI-based code generators attached to ASP.NET Core applications.

The primary API exposed by this package is an OpenApiReference item group that allows a user to define a reference to an OpenAPI document(s) that code generators can use:

<PropertyGroup>
  <OpenApiReference Include="mypath\openapi.json" />
</PropertyGroup>

There's an also an <OpenApiProjectReference> property that can be used to define the configuration options for a code generator.

This package is optimized for consumption by NSwag. Efforts to add Kiota support to the package in the past by @baywet have not been successful (see #41928).

In conversations with @RicoSuter it seems like the preference on the NSwag side is for users to invoke the code generation pipelines in NSwag via pre-existing NSwag tooling instead of the MSBuild-based middle line.

With all this in mind, given that:

• The package has received little in the way of updates over the past few years.
• The abstractions provided by the package don't scale well to non-NSwag code generators.
• Code generators provide their own CLIs and configuration options to drive code generation.

We should consider deprecating the package to reduce maintenance burden and provide a more straightforward story around integration ASP.NET Core with code generators in the future.

[captainsafia]

This was done in .NET 10 Preview 7.