Allow running benchmark with .NET 8 target framework

```
dotnet run -f net8.0 -c Release -- all
```

Author: jozefizso

Tests/NetOffice.Benchmarks/NetOffice.Benchmarks.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>net48;net10.0</TargetFrameworks>
+    <TargetFrameworks>net48;net8.0;net10.0</TargetFrameworks>
     <AssemblyTitle>NetOffice.Benchmarks</AssemblyTitle>
     <Description>Performance benchmarks for NetOffice Core</Description>
     <IsPackable>false</IsPackable>