Revert "Only lookup config.log_level for stdlib ::Logger. Closes #11665."

This reverts commit e0a521cfcd13e4d1f0ae8ab96004289e1c020f71.

Conflicts:
	railties/CHANGELOG.md

We expect loggers to quack like stdlib logger. If log4r needs different
level= assignment, using a Logger-quacking wrapper is the way to do it.

Fixes #14114.

railties/CHANGELOG.md

@@ -25,13 +25,6 @@
 
     *Cristian Mircea Messel*, *Chulki Lee*
 
-*   Only lookup `config.log_level` for stdlib `::Logger` instances.
-    Assign it as is for third party loggers like `Log4r::Logger`.
-
-    Fixes #13421.
-
-    *Yves Senn*
-
 *   The `Gemfile` of new applications depends on SDoc ~> 0.4.0.
 
     *Xavier Noria*

railties/lib/rails/application/bootstrap.rb

@@ -53,11 +53,7 @@ module Bootstrap
           logger
         end
 
-        if ::Logger === Rails.logger
-          Rails.logger.level = ActiveSupport::Logger.const_get(config.log_level.to_s.upcase)
-        else
-          Rails.logger.level = config.log_level
-        end
+        Rails.logger.level = ActiveSupport::Logger.const_get(config.log_level.to_s.upcase)
       end
 
       # Initialize cache early in the stack so railties can make use of it.

railties/test/application/configuration_test.rb

@@ -762,7 +762,7 @@ def index
       end
     end
 
-    test "lookup config.log_level with custom logger (stdlib Logger)" do
+    test "config.log_level with custom logger" do
       make_basic_app do |app|
         app.config.logger = Logger.new(STDOUT)
         app.config.log_level = :info
@@ -770,19 +770,6 @@ def index
       assert_equal Logger::INFO, Rails.logger.level
     end
 
-    test "assign log_level as is with custom logger (third party logger)" do
-      logger_class = Class.new do
-        attr_accessor :level
-      end
-      logger_instance = logger_class.new
-      make_basic_app do |app|
-        app.config.logger = logger_instance
-        app.config.log_level = :info
-      end
-      assert_equal logger_instance, Rails.logger
-      assert_equal :info, Rails.logger.level
-    end
-
     test "respond_to? accepts include_private" do
       make_basic_app