build: add net6 target

Using the SDK stlye project, it is fairly easy to target multiple
frameworks. For support here, a Crestron package update was necessary,
but targeting both .NET6 & .NET Framework 4.7.2 is possible. This should
allow us to support .NET 6 on 4-series processors, while still
supporting .NET Framework 4.7.2 on VC-4.

src/Pepperdash Core/PepperDash_Core.csproj

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <RootNamespace>PepperDash.Core</RootNamespace>
     <AssemblyName>PepperDashCore</AssemblyName>
-    <TargetFramework>net472</TargetFramework>
+    <TargetFrameworks>net472;net6</TargetFrameworks>
     <Deterministic>false</Deterministic>
     <NeutralLanguage>en</NeutralLanguage>
     <OutputPath>bin\$(Configuration)\</OutputPath>
@@ -30,7 +30,7 @@
     <Reference Include="System.Net.Http" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Crestron.SimplSharp.SDK.Library" Version="2.19.35" />
+    <PackageReference Include="Crestron.SimplSharp.SDK.Library" Version="2.20.42" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.2">
       <Aliases>Full</Aliases>
     </PackageReference>