Bump JetBrains.Annotations to version 2020.1.0

Author: drewnoakes

src/Directory.Build.props

@@ -6,7 +6,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="JetBrains.Annotations" Version="10.4.0" PrivateAssets="All" />
+    <PackageReference Include="JetBrains.Annotations" Version="2020.1.0" PrivateAssets="All" />
   </ItemGroup>
 
   <!-- Workaround for https://github.com/dotnet/sourcelink/issues/572 -->