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

(Currently, this patch will suppress `warning: logger was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.` message.)

Similar with GH-4411

Signed-off-by: Watson <watson1978@gmail.com>

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?