[tests] Switch to httpbin.org (#7556)

Context: 234cf2c

Commit 234cf2c updated the
`AndroidHandlerTestBase.Redirect_Without_Protocol_Works()` and
`AndroidHandlerTestBase.Redirect_POST_With_Content_Works()` tests to
use <httpbingo.org> to test HTTP redirect behavior.

Recently, <httpbingo.org> started returning HTTP-403 for our
redirection test, whereas we expected it to send HTTP-302:

	% curl -D - 'https://httpbingo.org/redirect-to?url=https://github.com/xamarin/xamarin-android'
	HTTP/2 403 
	access-control-allow-credentials: true
	access-control-allow-origin: *
	content-type: text/plain; charset=utf-8
	x-content-type-options: nosniff
	date: Wed, 16 Nov 2022 15:38:02 GMT
	content-length: 51
	server: Fly/5f2bf728 (2022-11-11)
	via: 2 fly.io
	fly-request-id: 01GJ0HE398V7F0BKFHY0D2N8YE-iad

	Forbidden redirect URL. Be careful with this link.

This caused the `Redirect_Without_Protocol_Works()` and
`Redirect_POST_With_Content_Works()` unit tests to start failing:

	System.Net.Http.HttpRequestException : Response status code does not indicate success: 403 (Forbidden).
	  at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode ()
	  at Xamarin.Android.NetTests.AndroidHandlerTestBase.Redirect_POST_With_Content_Works ()

Update the tests to *stop* using <httpbingo.org> and instead use
<httpbin.org>.  This allows our tests to work as expected:

	% curl -D - 'https://httpbin.org/redirect-to?url=https://github.com/xamarin/xamarin-android' 
	HTTP/2 302 
	date: Wed, 16 Nov 2022 15:38:28 GMT
	content-type: text/html; charset=utf-8
	content-length: 0
	location: https://github.com/xamarin/xamarin-android
	server: gunicorn/19.9.0
	access-control-allow-origin: *
	access-control-allow-credentials: true

Author: dellis1972

tests/Mono.Android-Tests/Xamarin.Android.Net/AndroidClientHandlerTests.cs

@@ -246,7 +246,7 @@ public void Property_Timeout_Works()
 		[Test]
 		public void Redirect_Without_Protocol_Works()
 		{
-			var requestURI = new Uri ("https://httpbingo.org/redirect-to?url=https://github.com/xamarin/xamarin-android");
+			var requestURI = new Uri ("https://httpbin.org/redirect-to?url=https://github.com/xamarin/xamarin-android");
 			var redirectedURI = new Uri ("https://github.com/xamarin/xamarin-android");
 			using (var c = new HttpClient (CreateHandler ())) {
 				var tr = ConnectIgnoreFailure (() => c.GetAsync (requestURI), out bool connectionFailed);
@@ -265,7 +265,7 @@ public void Redirect_Without_Protocol_Works()
 		[Test]
 		public void Redirect_POST_With_Content_Works ()
 		{
-			var requestURI = new Uri ("https://httpbingo.org/redirect-to?url=https://github.com/xamarin/xamarin-android");
+			var requestURI = new Uri ("https://httpbin.org/redirect-to?url=https://github.com/xamarin/xamarin-android");
 			var redirectedURI = new Uri ("https://github.com/xamarin/xamarin-android");
 			using (var c = new HttpClient (CreateHandler ())) {
 				var request = new HttpRequestMessage (HttpMethod.Post, requestURI);