The Microsoft.AspNetCore.Authentication package is now included in the Microsoft.AspNetCore.App metapackage.

It is automatically referenced if your project type is Microsoft.NET.Sdk.Web or you add a FrameworkReference. See Use ASP.NET Core APIs in a class library and this question for more details about adding a framework reference.

<ItemGroup>
  <FrameworkReference Include="Microsoft.AspNetCore.App" />
</ItemGroup>

Since you said your project is an ASP.Net WebApi project, I would guess that an explicit FrameworkReference would not be needed. All you may need to do is remove the old package reference.