Add bigdecimal to runtime dependency

[koic]

Motivation / Background

Follow up ruby/ruby@1c93288.

This PR adds bigdecimal to runtime dependency of Active Support to suppress the following Ruby 3.3.0dev's warning.

/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/bundler/gems/rails-a8871e6829e5/activesupport/lib/
active_support/core_ext/object/json.rb:5: warning: bigdecimal will be not part of the default gems since Ruby 3.4.0.
Add bigdecimal to your Gemfile. Also contact author of to add bigdecimal into its gemspec.

The grep yields the following results:

$ git grep 'require.*bigdecimal'
activejob/lib/active_job/arguments.rb:3:require "bigdecimal"
activejob/lib/active_job/serializers/big_decimal_serializer.rb:3:require "bigdecimal"
activejob/test/cases/argument_serialization_test.rb:3:require "bigdecimal"
activemodel/lib/active_model/type/decimal.rb:3:require "bigdecimal/util"
activemodel/lib/active_model/validations/numericality.rb:5:require "bigdecimal/util"
activemodel/test/cases/validations/numericality_validation_test.rb:8:require "bigdecimal"
activerecord/test/cases/adapters/sqlite3/quoting_test.rb:4:require "bigdecimal"
activerecord/test/cases/arel/visitors/to_sql_test.rb:4:require "bigdecimal"
activerecord/test/cases/migration_test.rb:5:require "bigdecimal/util"
activesupport/lib/active_support/core_ext/big_decimal/conversions.rb:3:require "bigdecimal"
activesupport/lib/active_support/core_ext/big_decimal/conversions.rb:4:require "bigdecimal/util"
activesupport/lib/active_support/core_ext/object/json.rb:5:require "bigdecimal"
activesupport/lib/active_support/message_pack/extensions.rb:3:require "bigdecimal"
activesupport/lib/active_support/xml_mini.rb:5:require "bigdecimal"
activesupport/lib/active_support/xml_mini.rb:6:require "bigdecimal/util"
activesupport/test/core_ext/hash_ext_test.rb:4:require "bigdecimal"
activesupport/test/core_ext/object/duplicable_test.rb:4:require "bigdecimal"
activesupport/test/hash_with_indifferent_access_test.rb:4:require "bigdecimal"
activesupport/test/json/encoding_test_cases.rb:3:require "bigdecimal"

By adding only to Active Support as a dependency, it should resolve the issue due to the dependency.

Detail

The warning is confirmed in the following step:

$ cat generic_main.rb
# frozen_string_literal: true

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"
end

require 'active_support'
require 'minitest/autorun'
# These gems will be bundled gems in Ruby 3.4
require 'bigdecimal'

Run generic_main.rb with Ruby 3.3.0dev below.

$ ruby -v
ruby 3.3.0dev (2023-08-25T17:47:04Z master 7d32011399) [x86_64-darwin22]

$ ruby generic_main.rb
Fetching gem metadata from https://rubygems.org/.......
Resolving dependencies...
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/bundler/gems/rails-a8871e6829e5/activesupport/lib/
active_support/core_ext/object/json.rb:5: warning: bigdecimal will be not part of the default gems since Ruby 3.4.0.
Add bigdecimal to your Gemfile. Also contact author of  to add bigdecimal into its gemspec.
Run options: --seed 39015

# Running:

Finished in 0.001313s, 0.0000 runs/s, 0.0000 assertions/s.
0 runs, 0 assertions, 0 failures, 0 errors, 0 skips

Additional information

It is essentially the same as #48907.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[zzak]

FYI: This error started re-appearing in CI on rubylang/ruby:master-nightly-jammy (3.3.0dev) builds:
https://buildkite.com/rails/rails/builds/102659#018c5108-8fcf-4cbb-b7d5-0986156576dd/1100-1105

[Earlopain]

Should this be backported to 7.0?

FYI: This error started re-appearing in CI on rubylang/ruby:master-nightly-jammy (3.3.0dev) builds:
https://buildkite.com/rails/rails/builds/102659#018c5108-8fcf-4cbb-b7d5-0986156576dd/1100-1105

I'd bet on this ruby/ruby@83bdf12, though I haven't checked if this is good now with a proper 3.3 release.