Railsチュートリアルの1.5.2でRubyのバージョンがHerokuに対応していないと言われた

IDEはRailsチュートリアルに則りAWS Cloud9のものを使用。
Gemfileの内容は以下のとおり。

Gemfile

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

ruby '2.6.3'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '6.0.3'
# Use Puma as the app server
gem 'puma', '4.3.6'
# Use SCSS for stylesheets
gem 'sass-rails', '5.1.0'
# Transpile app-like JavaScript. Read more: https://github.com/rails/webpacker
gem 'webpacker', '4.0.7'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '5.2.0'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '2.9.1'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use Active Model has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Active Storage variant
# gem 'image_processing', '~> 1.2'

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '1.4.5', require: false

group :development, :test do
  # Use sqlite3 as the database for Active Record
  gem 'sqlite3', '1.4.1'
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', '11.0.1', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  # Access an interactive console on exception pages or by calling 'console' anywhere in the code.
  gem 'web-console', '4.0.1'
  gem 'listen', '3.1.5'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring', '2.1.0'
  gem 'spring-watcher-listen', '2.0.1'
end

group :test do
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '3.28.0'
  gem 'selenium-webdriver', '3.142.4'
  # Easy installation and use of web drivers to run system tests with browsers
  gem 'webdrivers', '4.1.2'
end

group :production do
  gem 'pg', '1.1.4'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]'

これで以下のコマンドを実行した。

$ git push heroku master

すると以下のように表示された

...

remote:  !
remote:  !     The Ruby version you are trying to install does not exist on this stack.
remote:  !     
remote:  !     You are trying to install ruby-2.6.3 on heroku-20.
remote:  !     
remote:  !     Ruby ruby-2.6.3 is present on the following stacks:
remote:  !     
remote:  !     - cedar-14
remote:  !     - heroku-16
remote:  !     - heroku-18
remote:  !     
remote:  !     Heroku recommends you use the latest supported Ruby version listed here:
remote:  !     https://devcenter.heroku.com/articles/ruby-support#supported-runtimes
remote:  !     
remote:  !     For more information on syntax for declaring a Ruby version see:
remote:  !     https://devcenter.heroku.com/articles/ruby-versions
remote:  !

...

どうやらRalisチュートリアルで使っているRubyのバージョンがHerokuに対応していないので、ここに載っているバージョンを使ってほしいということらしい
https://devcenter.heroku.com/articles/ruby-support#supported-runtimes

早速以下のようにGemfileを書き換えた。
要するにrubyのバージョンを変えただけ。

Gemfile

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

ruby '2.6.6'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '6.0.3'
# Use Puma as the app server
gem 'puma', '4.3.6'
# Use SCSS for stylesheets
gem 'sass-rails', '5.1.0'
# Transpile app-like JavaScript. Read more: https://github.com/rails/webpacker
gem 'webpacker', '4.0.7'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '5.2.0'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '2.9.1'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use Active Model has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Active Storage variant
# gem 'image_processing', '~> 1.2'

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '1.4.5', require: false

group :development, :test do
  # Use sqlite3 as the database for Active Record
  gem 'sqlite3', '1.4.1'
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', '11.0.1', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  # Access an interactive console on exception pages or by calling 'console' anywhere in the code.
  gem 'web-console', '4.0.1'
  gem 'listen', '3.1.5'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring', '2.1.0'
  gem 'spring-watcher-listen', '2.0.1'
end

group :test do
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '3.28.0'
  gem 'selenium-webdriver', '3.142.4'
  # Easy installation and use of web drivers to run system tests with browsers
  gem 'webdrivers', '4.1.2'
end

group :production do
  gem 'pg', '1.1.4'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

あとは下記のコマンドを実行してデプロイすればOK

$ bundle update
$ git commit -a -m "RubyのバージョンをHerokuに対応している2.6.6に変更"
$ git push heroku master

Railsチュートリアルの1.3.1ではGemfileのRubyのバージョンを2.6.3としていたが、どこかで変更しているのを私が見落としたのだろうか？