title: After update to VS 16.9.0: Feature 'init-only setters' is not available in C# 7.3. Please use language version 9.0 or greater
body: <p>We have a project that targets <code>netstandard2.0</code> and that project uses the NuGet package <a href="https://github.com/JoshClose/CsvHelper" rel="noreferrer">CsvHelper</a>, that is also available as <code>netstandard2.0</code>.</p>
  <p>This project contains the class <code>CsvConfiguration</code> that is build as:</p>
  <pre><code>public class CsvConfiguration
  {
      public virtual string Delimiter { get; init; }
  }
  </code></pre>
  <p>And we use this library without any problems in the past. After updating Visual Studio from 16.8.6 to 16.9.0 the above mentioned error message occurs on a build:</p>
  <pre><code>error CS8370: Feature 'init-only setters' is not available in C# 7.3. Please use language version 9.0 or greater.
  </code></pre>
  <p>While I understand why this error would occur, if I would use this feature in my own code, I don't know why this happens if a third party library uses it, that is declared to match my framework version.</p>
  <p>Is this a <em>bug</em> in the library or in Visual Studio? Cause in VS 16.8.6 it compiles fine and in VS 16.9.0 it fails.</p>
  <p>I know, an <em>easy</em> workaround would be to add a <code>&lt;LangVersion&gt;</code> entry within our .csproj file, but is this the recommended way?</p>