update coverlet.msbuild 3.1.0 (#1273)

Author: SimonCropp

test/Directory.Build.props

@@ -19,7 +19,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="coverlet.msbuild" Version="3.0.3" />
+    <PackageReference Include="coverlet.msbuild" Version="3.1.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
     <PackageReference Include="NSubstitute" Version="4.2.2" />
     <PackageReference Include="FluentAssertions" Version="6.1.0" />