Updated Benchmarks to target net8.0

Author: jstedfast

Benchmarks/Benchmarks.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks Condition="'$(TestFrameworks)'==''">net6.0</TargetFrameworks>
+    <TargetFrameworks Condition="'$(TestFrameworks)'==''">net8.0</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <DefineConstants Condition=" '$(MonoRuntime)' == 'true' ">$(DefineConstants);MONO</DefineConstants>
     <AssemblyOriginatorKeyFile>..\MimeKit\mimekit.snk</AssemblyOriginatorKeyFile>