[rubygems/rubygems] Migrate mirror probe specs to use the compact index API

Could potentially fix some flakies we're using and make the specs more
"modern" and simplifies them because less fallbacks are involved.

ruby/rubygems@30da9a1a93

Author: deivid-rodriguez

spec/bundler/install/gems/mirror_probe_spec.rb

@@ -105,13 +105,13 @@ def setup_server
     @server_port = find_unused_port
     @server_uri = "http://#{host}:#{@server_port}"
 
-    require_relative "../../support/artifice/endpoint"
+    require_relative "../../support/artifice/compact_index"
     require_relative "../../support/silent_logger"
 
     require "rackup/server"
 
     @server_thread = Thread.new do
-      Rackup::Server.start(app: Endpoint,
+      Rackup::Server.start(app: CompactIndexAPI,
                            Host: host,
                            Port: @server_port,
                            server: "webrick",