Fix (Http)WebRequest (Begin|End)GetResponse samples (dotnet#11030)

These samples were not handling the `asyncResult.CompletedSynchronously == true` case correctly: changing the sample code to use smaller buffer leads to stack overflow because of recursion on synchronous completions.

Besides the fix there the PR adds some consolidation:
- Cleanup and (somewhat) modernize the sample code
- Remove the Timer-based custom timeout implementation, use `WebRequest.Timeout` instead
- Collapse various samples into one sample
  - Unifiy the `HttpWebRequest` and `WebRequest` samples since they are practically identical
  - Delete CPP and VB samples given we don't have the capacity and the intention to maintain them.

Author: antonfirsov

snippets/cpp/VS_Snippets_Remoting/System.Net.HttpWebRequest.BeginGetResponse/CPP/begingetresponse.cpp

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <OutputType>Library</OutputType>
+    <OutputType>Exe</OutputType>
     <TargetFramework>net6.0</TargetFramework>
   </PropertyGroup>