Disable default behavior of frozen_string_literal for Ruby 3.4+

According to https://bugs.ruby-lang.org/issues/20205,
it will enable `# frozen_string_literal: true` by default in the future Ruby.

This changing will affect many Fluentd plugins, and they will require to be deal with it properly.
But I think it is too tough.

So, this patch will set `--disable=frozen_string_literal` as launch option to disable default behavior for `# frozen_string_literal: true` in future Ruby.

GitHub: fluentGH-4585
Signed-off-by: Watson <[email protected]>

Rakefile

@@ -43,7 +43,7 @@ Rake::TestTask.new(:base_test) do |t|
                  end
   t.verbose = true
   t.warning = true
-  t.ruby_opts = ["-Eascii-8bit:ascii-8bit"]
+  t.ruby_opts = ["-Eascii-8bit:ascii-8bit --disable=frozen_string_literal"]
 end
 
 task :parallel_test do

lib/fluent/supervisor.rb

@@ -1019,6 +1019,8 @@ def build_spawn_command
         fluentd_spawn_cmd = [ServerEngine.ruby_bin_path]
       end
 
+      fluentd_spawn_cmd << '--disable=frozen_string_literal'
+
       rubyopt = ENV['RUBYOPT']
       if rubyopt
         encodes, others = rubyopt.split(' ').partition { |e| e.match?(RUBY_ENCODING_OPTIONS_REGEX) }