Add deprecation warning to Microsoft.Extensions.ApiDescription.Client MSBuild package

[Copilot]

This PR adds a compile-time deprecation warning to the Microsoft.Extensions.ApiDescription.Client MSBuild package to inform users that the package is deprecated and will be removed in a future release.

Changes Made

• Added a deprecation warning target to build/Microsoft.Extensions.ApiDescription.Client.props
• The warning appears during restore, build, and compile phases for any project that references this package
• Works for both single-targeting and multi-targeting scenarios
• Preserves all existing MSBuild functionality

Warning Message

When developers build projects that reference this package, they will now see:

warning : The 'Microsoft.Extensions.ApiDescription.Client' package is deprecated and will be removed in a future release. Please consider migrating to alternative solutions for OpenAPI client code generation.

Implementation Details

The warning is implemented as an MSBuild target that executes before the BeforeCompile, Build, and Restore targets:

<Target Name="_WarnMicrosoftExtensionsApiDescriptionClientDeprecated" BeforeTargets="BeforeCompile;Build;Restore">
  <Warning Text="The 'Microsoft.Extensions.ApiDescription.Client' package is deprecated and will be removed in a future release. Please consider migrating to alternative solutions for OpenAPI client code generation." />
</Target>

Testing

• Verified the warning appears correctly during build for both single-target and multi-target projects
• Confirmed all existing MSBuild tasks and properties continue to work as expected
• The warning does not break the build process (it's a warning, not an error)

This change provides clear communication to developers about the package's deprecation status while maintaining backward compatibility.

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[DamianEdwards]

Just verifying, this can be suppressed with <NoWarn>$(NoWarn);ASPDEPR001</NoWarn> in the project file, right?

[captainsafia]

Yep -- NoWarn will work now that we have a code for this.