http_server: stop fallback to WEBrick (#4899)

**Which issue(s) this PR fixes**: 
* Partially Fixes #4648

**What this PR does / why we need it**: 
Make `http_server` plugin helper stop the fallback to WEBrick.

WEBrick is no longer recommended for production use.

We don't need this feature because it was for supporting Ruby < 2.3.
(#2447)

**Docs Changes**:

https://docs.fluentd.org/plugin-helper-overview/api-plugin-helper-http_server
=> fluent/fluentd-docs-gitbook#575

**Release Note**: 
The same as the title.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

fluentd.gemspec

@@ -35,6 +35,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency("webrick", ["~> 1.4"])
   gem.add_runtime_dependency("zstd-ruby", ["~> 1.5"])
   gem.add_runtime_dependency("uri", '~> 1.0')
+  gem.add_runtime_dependency("async-http", "~> 0.86")
 
   # gems that aren't default gems as of Ruby 3.4
   gem.add_runtime_dependency("base64", ["~> 0.2"])
@@ -68,7 +69,6 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency("test-unit", ["~> 3.3"])
   gem.add_development_dependency("test-unit-rr", ["~> 1.0"])
   gem.add_development_dependency("oj", [">= 2.14", "< 4"])
-  gem.add_development_dependency("async-http", "~> 0.86")
   gem.add_development_dependency("console", "~> 1.30")
   gem.add_development_dependency("aws-sigv4", ["~> 1.8"])
   gem.add_development_dependency("aws-sdk-core", ["~> 3.191"])

lib/fluent/plugin_helper/http_server.rb

@@ -14,16 +14,9 @@
 #    limitations under the License.
 #
 
-begin
-  require 'async'
-  require 'fluent/plugin_helper/http_server/server'
-rescue LoadError => _
-  require 'fluent/plugin_helper/http_server/compat/server'
-  Fluent::PluginHelper::HttpServer::Server = Fluent::PluginHelper::HttpServer::Compat::Server
-end
-
 require 'fluent/plugin_helper/thread'
 require 'fluent/plugin_helper/server' # For Server::ServerTransportParams
+require 'fluent/plugin_helper/http_server/server'
 require 'fluent/plugin_helper/http_server/ssl_context_builder'
 
 module Fluent