Enable AggressiveAttributeTrimming (#13523)

This enables removing all kinds of infrastructure attributes for example SupportedOSPlatformAttribute (like old XI did).

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -134,6 +134,7 @@
 		<UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == '' And '$(_BundlerDebug)' == 'true'">false</UseSystemResourceKeys>
 		<UseNativeHttpHandler Condition="'$(_PlatformName)' != 'macOS' And '$(UseNativeHttpHandler)' == ''">true</UseNativeHttpHandler>
 		<AutoreleasePoolSupport Condition="'$(AutoreleasePoolSupport)' == ''">true</AutoreleasePoolSupport>
+		<_AggressiveAttributeTrimming Condition="'$(_AggressiveAttributeTrimming)' == ''">true</_AggressiveAttributeTrimming>		
 
 		<!-- We don't need to generate reference assemblies for apps or app extensions -->
 		<ProduceReferenceAssembly Condition="'$(ProduceReferenceAssembly)' == '' And ('$(OutputType)' == 'Exe' Or '$(IsAppExtension)' == 'true')">false</ProduceReferenceAssembly>