RailsでBigDecimal.new is deprecated; use BigDecimal() method instead.のエラーを解決した。

なぜこのエラーが出るのか

内容的には BigDecimal.new は非推奨だから BigDecimal() 使えよってことです。
ruby v2.6 から BigDecimal.new は非推奨になっています。

僕の場合は、matcherがBigDecimal.newでした。

shoulda/matchers/active_model/validate_inclusion_of_matcher.rb:273:
warning: BigDecimal.new is deprecated; use BigDecimal() method instead.

これにぴったりのissueを見つけました。
https://github.com/thoughtbot/shoulda-matchers/pull/1070

解決方法

今のgemfileにあるmatcher

group :test do
  gem 'shoulda-matchers',
    git: 'https://github.com/thoughtbot/shoulda-matchers.git',
    branch: 'rails-5'

公式を見ると、

 gem 'shoulda-matchers', '~> 4.0'

になっているから、それでbundle installをする。

https://github.com/thoughtbot/shoulda-matchers

group :test do
  gem 'shoulda-matchers', '~> 4.0'
end

rspec
....*****....................

Pending: (Failures listed here are expected and do not affect your suite's status)

  1) Comment add some examples to (or delete) /myapp/spec/models/comment_spec.rb
     # Not yet implemented
     # ./spec/models/comment_spec.rb:4

  2) Favorite add some examples to (or delete) /myapp/spec/models/favorite_spec.rb
     # Not yet implemented
     # ./spec/models/favorite_spec.rb:4

  3) Like add some examples to (or delete) /myapp/spec/models/like_spec.rb
     # Not yet implemented
     # ./spec/models/like_spec.rb:4

  4) PostTag add some examples to (or delete) /myapp/spec/models/post_tag_spec.rb
     # Not yet implemented
     # ./spec/models/post_tag_spec.rb:4

  5) Tag add some examples to (or delete) /myapp/spec/models/tag_spec.rb
     # Not yet implemented
     # ./spec/models/tag_spec.rb:4

Finished in 12.14 seconds (files took 3.48 seconds to load)
29 examples, 0 failures, 5 pending

無事warning: BigDecimal.new is deprecated; のエラーが消えていた。