update to .net framework 4.8

Source/BingMapsSDSToolkit.Standard/BingMapsSDSToolkit.Standard.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.1</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <RootNamespace>BingMapsSDSToolkit</RootNamespace>
     <AssemblyName>BingMapsSDSToolkit</AssemblyName>
     <Version>1.0.5</Version>