Run unit tests only on .NET 8.

I am not in the mood to figure out how to make the azure pipeline run 7.0 and also 8.0.

Author: StringEpsilon

PhpSerializerNET.Test/PhpSerializerNET.Test.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<PropertyGroup>
-		<TargetFrameworks>net7.0;net8.0</TargetFrameworks>
+		<TargetFramework>net8.0</TargetFramework>
 		<IsPackable>false</IsPackable>
 	</PropertyGroup>
 	<ItemGroup>