Upgrade PixiEditor.Numerics to .NET 8

src/PixiEditor.Numerics/PixiEditor.Numerics.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>netstandard2.1</TargetFramework>
+        <TargetFramework>net8.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
         <LangVersion>default</LangVersion>