Add logger gem as dependency for Ruby 3.5

logger gem was marked as bundled gem for Ruby 3.5.
Therefore, it will not work on Ruby 3.5 unless it is added as a dependency.
Ref. ruby/ruby@d7e558e

Similar with fluentGH-4411

fluentd.gemspec

@@ -39,6 +39,9 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency("csv", ["~> 3.2"])
   gem.add_runtime_dependency("drb", ["~> 2.2"])
 
+  # gems that aren't default gems as of Ruby 3.5
+  gem.add_runtime_dependency("logger", ["~> 1.6"])
+
   # build gem for a certain platform. see also Rakefile
   fake_platform = ENV['GEM_BUILD_FAKE_PLATFORM'].to_s
   gem.platform = fake_platform unless fake_platform.empty?