[mdoc] Set RollForward to Major (#671)

Context: https://learn.microsoft.com/en-us/dotnet/core/project-sdk/msbuild-props#rollforward

Allows the `net6.0` version of mdoc to run against newer dotnet runtime
versions, rather than requiring `net6.0` to be installed.

Co-authored-by: Min Huang <[email protected]>

Author: pjcollins

mdoc/mdoc.csproj

@@ -7,6 +7,7 @@
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <ProduceReferenceAssembly>false</ProduceReferenceAssembly>
     <AutoGenerateBindingRedirects>false</AutoGenerateBindingRedirects>
+    <RollForward>Major</RollForward>
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(TargetFramework)' == 'net471' ">