[build] require .NET framework >= 4.7 (#3164)

.nuspec/Xamarin.Forms.targets

@@ -54,6 +54,15 @@
 			Condition="'$(IsXBuild)' == 'true'" />
 	</Target>
 
+	<Target Name="_ValidateNETFrameworkVersion"
+			BeforeTargets="_CheckForInvalidConfigurationAndPlatform"
+			Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' AND '$(TargetFrameworkVersion.Substring(1))' &lt; '4.7' ">
+		<Error
+			Text="XF requires .NETFramework >= v4.7. You have '$(TargetFrameworkVersion)'"
+			Code="XF004"
+			 />
+	</Target>
+
 	<!-- XamlG -->
 	<Target Name="UpdateDesignTimeXaml" DependsOnTargets="XamlG"/>