Improve compatibility between Logger and ActiveSupport::BroadcastLogger

The usage of `dispatch` in all logging methods causes common usages such
as `logger.info` to return an array of loggers, making it unsafe for an
application to upgrade to Rails 7.1.

Returning `nil` is more efficient, and is the default behavior when
using `Logger`.

activesupport/lib/active_support/broadcast_logger.rb

@@ -229,6 +229,7 @@ def initialize_copy(other)
     private
       def dispatch(&block)
         @broadcasts.each { |logger| block.call(logger) }
+        true
       end
 
       def method_missing(name, ...)

activesupport/test/broadcast_logger_test.rb

@@ -302,6 +302,11 @@ def info(msg, &block)
       assert_same logger, broadcast_logger.broadcasts.sole
     end
 
+    test "logging has no return value" do
+      assert_equal true, @logger.info("Hello")
+      assert_equal true, @logger.error("Hello")
+    end
+
     class CustomLogger
       attr_reader :adds, :closed, :chevrons
       attr_accessor :level, :progname, :formatter, :local_level