AddDefaultIdentity已在 ASP.NET Core 2.1 中引進

https://docs.microsoft.com/zh-tw/aspnet/core/security/authentication/identity?view=aspnetcore-3.1&tabs=visual-studio#adddefaultidentity-and-addidentity
AddDefaultIdentity已在 ASP.NET Core 2.1 中引進。 呼叫AddDefaultIdentity類似于呼叫下列內容：AddIdentity、AddDefaultUI、AddDefaultTokenProviders

原始碼
https://github.com/aspnet/Identity/blob/c7276ce2f76312ddd7fccad6e399da96b9f6fae1/src/UI/IdentityServiceCollectionUIExtensions.cs#L49

ref:
https://docs.microsoft.com/zh-tw/aspnet/core/security/authorization/secure-data?view=aspnetcore-3.1