Update test dependencies

spec/bundler/install/gems/dependency_api_fallback_spec.rb

@@ -15,8 +15,10 @@ RSpec.describe "gemcutter's dependency API" do
       # mustermann depends on URI::RFC2396_PARSER behavior
       URI.parser = URI::RFC2396_PARSER if URI.respond_to?(:parser=)
 
+      require "rackup/server"
+
       @t = Thread.new do
-        server = Rack::Server.start(app: EndpointTimeout,
+        server = Rackup::Server.start(app: EndpointTimeout,
                                       Host: "0.0.0.0",
                                       Port: port,
                                       server: "webrick",

spec/bundler/realworld/gemfile_source_header_spec.rb

@@ -39,8 +39,10 @@ RSpec.describe "fetching dependencies with a mirrored source", realworld: true d
 
     require_relative "../support/artifice/endpoint_mirror_source"
 
+    require "rackup/server"
+
     @t = Thread.new do
-      Rack::Server.start(app: EndpointMirrorSource,
+      Rackup::Server.start(app: EndpointMirrorSource,
                            Host: "0.0.0.0",
                            Port: @port,
                            server: "webrick",

spec/bundler/realworld/mirror_probe_spec.rb

@@ -112,8 +112,10 @@ RSpec.describe "fetching dependencies with a not available mirror", realworld: t
 
     require_relative "../support/artifice/endpoint"
 
+    require "rackup/server"
+
     @server_thread = Thread.new do
-      Rack::Server.start(app: Endpoint,
+      Rackup::Server.start(app: Endpoint,
                            Host: host,
                            Port: @server_port,
                            server: "webrick",

tool/bundler/test_gems.rb

@@ -2,12 +2,13 @@
 
 source "https://rubygems.org"
 
-gem "rack", "~> 2.0"
+gem "rack", "~> 3.0"
+gem "rackup", "~> 2.1"
 gem "base64"
-gem "webrick", "1.7.0"
-gem "rack-test", "~> 1.1"
+gem "webrick", "~> 1.8"
+gem "rack-test", "~> 2.1"
 gem "compact_index", "~> 0.15.0"
-gem "sinatra", "~> 3.0"
+gem "sinatra", "~> 4.0"
 gem "rake", "~> 13.1"
 gem "builder", "~> 3.2"
 gem "rb_sys"