updated test project csproj

Author: Petr Sramek

tests/PolylineAlgorithm.Tests/PolylineAlgorithm.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFrameworks>net5.0;net6.0;net7.0;net8.0;net9.0</TargetFrameworks>
+		<TargetFramework>net9.0</TargetFramework>
 		<LangVersion>13.0</LangVersion>
 		<Nullable>enable</Nullable>
 		<ImplicitUsings>enable</ImplicitUsings>