upgrade JetBrains.Annotations

src/Directory.Build.props

@@ -25,7 +25,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
-    <PackageReference Include="JetBrains.Annotations" Version="2019.1.3" PrivateAssets="All" />
+    <PackageReference Include="JetBrains.Annotations" Version="2020.1.0" PrivateAssets="All" />
   </ItemGroup>
 
 </Project>