Fix SYSLIB0014 warning related to use of WebRequest-related types

Replace use of obsolete WebRequest-related types with that of HttpClient-related ones.

This was done in several steps, ultimately squashed into a single commit:

- Claude did the heavy lifting
- Manual tweaks were made afterwards, to make code changes a little cleaner

This introduces breaking Changes:
- ⚠ Users can no longer override GetResponse(), GetResponseAsync(), or GetRequestStream() methods
- ✅ Migration path: Use IHttpMessageHandler interface instead

Author: louis-z

NGitLab.Mock/NGitLab.Mock.csproj

@@ -22,4 +22,4 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
   </ItemGroup>
-</Project>
+</Project>

NGitLab.Tests/Docker/GitLabTestContext.cs

@@ -5,7 +5,6 @@
 using System.Globalization;
 using System.IO;
 using System.Linq;
-using System.Net;
 using System.Net.Http;
 using System.Runtime.InteropServices;
 using System.Security.Cryptography;
@@ -74,7 +73,7 @@ public static async Task<GitLabTestContext> CreateAsync()
 
     public IGitLabClient Client { get; }
 
-    public WebRequest LastRequest => _customRequestOptions.AllRequests[_customRequestOptions.AllRequests.Count - 1];
+    public HttpRequestMessage LastRequest => _customRequestOptions.AllRequests[_customRequestOptions.AllRequests.Count - 1];
 
     private static bool IsUnique(string str)
     {