Add logger gem as dependency for Ruby 3.5 (#4589)

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>

Author: Watson1978

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?