Use JSON.generate instead of Yajl.dump

Recently, Ruby's JSON has huge improvement and it is much faster.
Ref. https://byroot.github.io/ruby/json/2024/12/15/optimizing-ruby-json-part-1.html

So, this PR will change yajl-ruby to Ruby's JSON.

### envronment
- Ruby: 3.4.4
- OS: Linux 6.14.6-2
- Compiler: gcc 15.1.1

### benchmark

```ruby
require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
  gem 'json'
  gem 'yajl-ruby'
end

require 'json'
require 'yajl'
require 'yajl/version'

puts "* JSON version: #{JSON::VERSION}"
puts "* Yajl version: #{Yajl::VERSION}"

json = '{"a":"Alpha","b":true,"c":12345,"d":[true,[false,[-123456789,null],3.9676,["Something else.",false],null]],"e":{"zero":null,"one":1,"two":2,"three":[3],"four":[0,1,2,3,4]},"f":null,"h":{"a":{"b":{"c":{"d":{"e":{"f":{"g":null}}}}}}},"i":[[[[[[[null]]]]]]]}'
data = JSON.parse(json)

Benchmark.ips do |x|
  x.report('Yajl.dump') { Yajl.dump(data) }
  x.report('JSON.generate') { JSON.generate(data) }

  x.compare!
end
```

### result
```
$ ruby tmp.rb
* JSON version: 2.12.2
* Yajl version: 1.4.3
ruby 3.4.4 (2025-05-14 revision a38531fd3f) +PRISM [x86_64-linux]
Warming up --------------------------------------
           Yajl.dump    34.964k i/100ms
       JSON.generate   120.598k i/100ms
Calculating -------------------------------------
           Yajl.dump    345.056k (± 2.8%) i/s    (2.90 μs/i) -      1.748M in   5.070452s
       JSON.generate      1.212M (± 1.6%) i/s  (824.89 ns/i) -      6.150M in   5.074868s

Comparison:
       JSON.generate:  1212286.6 i/s
           Yajl.dump:   345056.0 i/s - 3.51x  slower
```

Author: Watson1978

LICENSE-3rdparty.csv

@@ -1,5 +1,4 @@
 Component,Origin,License
-plugin,github.com/brianmario/yajl-ruby,MIT
 plugin,github.com/bundler/bundler,MIT
 plugin,github.com/ruby/openssl,BSD-2-Clause
 plugin,github.com/ruby/rake,MIT

fluent-plugin-datadog.gemspec

@@ -29,7 +29,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 2.1"
   spec.add_development_dependency "test-unit", '~> 3.1'
   spec.add_development_dependency "rake", "~> 12.0"
-  spec.add_development_dependency "yajl-ruby", "~> 1.2"
   spec.add_development_dependency 'webmock', "~> 3.6.0"
 
   spec.metadata      = {

lib/fluent/plugin/out_datadog.rb

@@ -5,7 +5,7 @@
 
 require "socket"
 require "openssl"
-require "yajl"
+require "json"
 require "zlib"
 require "fluent/plugin/output"
 
@@ -118,10 +118,10 @@ def format(tag, time, record)
     # is compatible with Time.at below.
     record = enrich_record(tag, time.to_f, record)
     if @use_http
-      record = Yajl.dump(record)
+      record = JSON.generate(record)
     else
       if @use_json
-        record = "#{api_key} #{Yajl.dump(record)}"
+        record = "#{api_key} #{JSON.generate(record)}"
       else
         record = "#{api_key} #{record}"
       end