Mark tests obsolete and remove for net8

Author: FObermaier

test/ProjNet.Tests/ProjNET.Tests.csproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net6</TargetFramework>
+    <TargetFrameworks>net6;net8</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <NoWarn>1701;1702;1591</NoWarn>
   </PropertyGroup>

test/ProjNet.Tests/Serialization/BaseSerializationTest.cs

@@ -1,16 +1,20 @@
-using System.IO;
+using System;
+using System.IO;
+#if !NET7_0_OR_GREATER
 using System.Runtime.Serialization;
 using System.Runtime.Serialization.Formatters.Binary;
 
 namespace ProjNET.Tests.Serialization
 {
     public class BaseSerializationTest
     {
+        [Obsolete("ISerializable is deprecated")]
         public IFormatter GetFormatter()
         {
             return new BinaryFormatter();
         }
 
+        [Obsolete("ISerializable is deprecated")]
         public static T SanD<T>(T instance, IFormatter formatter)
         {
             using (var ms = new MemoryStream())
@@ -21,4 +25,5 @@ public static T SanD<T>(T instance, IFormatter formatter)
             }
         }
     }
-}
+}
+#endif

test/ProjNet.Tests/Serialization/CoordinateSystemsProjectionsTest.cs

@@ -1,11 +1,14 @@
 using NUnit.Framework;
 using ProjNet.CoordinateSystems;
+using System;
 
 namespace ProjNET.Tests.Serialization
 {
-    public class CoordinateSystemsProjectionsTest : BaseSerializationTest
+    public class CoordinateSystemsProjectionsTest
+#if !NET7_0_OR_GREATER
+        : BaseSerializationTest
     {
-        [Test]
+        [Test, Obsolete("ISerializable is deprecated")]
         public void TestProjectionParameterSet() 
         {
             var ps = new ProjNet.CoordinateSystems.Projections.ProjectionParameterSet(
@@ -20,6 +23,9 @@ public void TestProjectionParameterSet()
 
             Assert.AreEqual(ps, psD);
         }
+#else
+    {
+#endif
 
         [Test]
         public void CreateTransformationFromCoordinateSystemDeserializedFromWKT()