[ruby/json] Use JSON.generate instead of Object#to_json

[p8]

JSON.generate is faster than calling Object#to_json.

require 'benchmark/ips'
require 'json'

Benchmark.ips do |x|
  x.config(warmup: 2, time: 5)

  x.report("to_json symbol") do
    { message: 'Hello, World!' }.to_json
  end

  x.report("to_json string") do
    { 'message' => 'Hello, World!' }.to_json
  end

  x.report("JSON.generate symbol") do
    JSON.generate({ message: 'Hello, World!' })
  end

  x.report("JSON.generate string") do
    JSON.generate({ 'message' => 'Hello, World!' })
  end

  x.compare!
end

ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [arm64-darwin24]
Warming up --------------------------------------
      to_json symbol   330.723k i/100ms
      to_json string   325.411k i/100ms
JSON.generate symbol   490.246k i/100ms
JSON.generate string   495.752k i/100ms
Calculating -------------------------------------
      to_json symbol      3.219M (± 8.2%) i/s  (310.65 ns/i) -     16.205M in   5.097780s
      to_json string      3.250M (± 3.5%) i/s  (307.67 ns/i) -     16.271M in   5.012989s
JSON.generate symbol      4.966M (± 1.5%) i/s  (201.35 ns/i) -     25.003M in   5.035419s
JSON.generate string      4.973M (± 2.1%) i/s  (201.10 ns/i) -     25.283M in   5.086778s

Comparison:
JSON.generate string:  4972636.6 i/s
JSON.generate symbol:  4966478.7 i/s - same-ish: difference falls within error
      to_json string:  3250251.2 i/s - 1.53x  slower
      to_json symbol:  3219106.9 i/s - 1.54x  slower