Add .net 8 multitarget (#17452)

src/Browser/Avalonia.Browser/Avalonia.Browser.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>$(AvsCurrentBrowserTargetFramework)</TargetFramework>
+    <TargetFrameworks>$(AvsCurrentTargetFramework);$(AvsCurrentBrowserTargetFramework)</TargetFrameworks>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>