Fix the use of deprecated C API ruby/ruby#15447

Last usage was removed in a26f528
#15447

The old API was removed in ruby/ruby#15447 and the change will be released in ruby-4.1.
Ruby-4.0 still supports the old untyped API.

This commit replaces the `yajl-ruby` dependency with Ruby's standard
`json` gem for the following reasons:

1. Future Compatibility: `yajl-ruby` is no longer actively maintained
and relies on internal C APIs that are removed in Ruby 4.1. Migrating to
the standard `json` gem ensures the plugin can continue to be built and
used in future Ruby environments. Ref.
ruby/ruby#15447,
brianmario/yajl-ruby#233
2. Performance: The standard `json` gem has seen significant
improvements in recent updates and now outperforms `yajl-ruby`.

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

Benchmark.ips do |x|
  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]]]]]]]}'

  x.report('Yajl.load') {
    Yajl.load(json)
  }
  x.report('JSON.parse') {
    JSON.parse(json)
  }

  x.compare!
end
```

```
ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]
Warming up --------------------------------------
           Yajl.load    20.019k i/100ms
          JSON.parse    80.144k i/100ms
Calculating -------------------------------------
           Yajl.load    202.875k (± 0.2%) i/s    (4.93 μs/i) -      1.021M in   5.032515s
          JSON.parse    808.103k (± 0.2%) i/s    (1.24 μs/i) -      4.087M in   5.057949s

Comparison:
JSON.parse:   808103.0 i/s
 Yajl.load:   202874.5 i/s - 3.98x  slower
```

The old API was removed in ruby/ruby#15447 and the change will be released in ruby-4.1.
Ruby-4.0 still supports the old untyped API.

This commit replaces the `yajl-ruby` dependency with Ruby's standard `json` gem for the following reasons:

1. Future Compatibility: `yajl-ruby` is no longer actively maintained and relies on internal C APIs that are removed in Ruby 4.1.
   Migrating to the standard `json` gem ensures the plugin can continue to be built and used in future Ruby environments.
   Ref. ruby/ruby#15447, brianmario/yajl-ruby#233
2. Performance: The standard `json` gem has seen significant improvements in recent updates and now outperforms `yajl-ruby`.

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

Benchmark.ips do |x|
  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]]]]]]]}'

  x.report('Yajl.load') {
    Yajl.load(json)
  }
  x.report('JSON.parse') {
    JSON.parse(json)
  }

  x.compare!
end
```

```
ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]
Warming up --------------------------------------
           Yajl.load    20.019k i/100ms
          JSON.parse    80.144k i/100ms
Calculating -------------------------------------
           Yajl.load    202.875k (± 0.2%) i/s    (4.93 μs/i) -      1.021M in   5.032515s
          JSON.parse    808.103k (± 0.2%) i/s    (1.24 μs/i) -      4.087M in   5.057949s

Comparison:
JSON.parse:   808103.0 i/s
 Yajl.load:   202874.5 i/s - 3.98x  slower
```

This commit replaces the `yajl-ruby` dependency with Ruby's standard `json` gem for the following reasons:

1. Future Compatibility: `yajl-ruby` is no longer actively maintained and relies on internal C APIs that are removed in Ruby 4.1.
   Migrating to the standard `json` gem ensures the plugin can continue to be built and used in future Ruby environments.
   Ref. ruby/ruby#15447, brianmario/yajl-ruby#233
2. Performance: The standard `json` gem has seen significant improvements in recent updates and now outperforms `yajl-ruby`.

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

Benchmark.ips do |x|
  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]]]]]]]}'

  x.report('Yajl.load') {
    Yajl.load(json)
  }
  x.report('JSON.parse') {
    JSON.parse(json)
  }

  x.compare!
end
```

```
ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]
Warming up --------------------------------------
           Yajl.load    20.019k i/100ms
          JSON.parse    80.144k i/100ms
Calculating -------------------------------------
           Yajl.load    202.875k (± 0.2%) i/s    (4.93 μs/i) -      1.021M in   5.032515s
          JSON.parse    808.103k (± 0.2%) i/s    (1.24 μs/i) -      4.087M in   5.057949s

Comparison:
JSON.parse:   808103.0 i/s
 Yajl.load:   202874.5 i/s - 3.98x  slower
```