Use the condition guards for xplat builds

Author: Perksey

sources/Windowing/Windowing/Silk.NET.Windowing.csproj

@@ -1,7 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net8.0;net9.0;net8.0-android;net9.0-android;net8.0-ios;net9.0-ios</TargetFrameworks>
+        <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
+        <TargetFrameworks Condition="$(SilkAndroidDisabled)' != 'true'">$(TargetFrameworks);net8.0-android;net9.0-android</TargetFrameworks>
+        <TargetFrameworks Condition="$(SilkiOSDisabled)' != 'true'">$(TargetFrameworks);net8.0-ios;net9.0-ios</TargetFrameworks>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
         <NoWarn>0067</NoWarn> <!-- TODO -->