Ruby on rails で作ったappをherokuでデプロイする方法

Question

$ git push heroku main
でプッシュしようとしても、しばらくロードした後、

remote: !
remote: ! Could not detect rake tasks
remote: ! ensure you can run $ bundle exec rake -P against your app
remote: ! and using the production group of your Gemfile.
remote: ! Activating bundler (2.1.4) failed:
remote: ! Could not find 'bundler' (2.1.4) required by your /tmp/build_d44822c1/Gemfile.lock.
remote: ! To update to the latest version installed on your system, run bundle update --bundler.
remote: ! To install the missing version, run gem install bundler:2.1.4
remote: ! Checked in 'GEM_PATH=/tmp/build_d44822c1/vendor/bundle/ruby/2.6.0', execute gem env for more information
remote: !
remote: ! To install the version of bundler this project requires, run gem install bundler -v '2.1.4'
remote: !

というエラーが発生してしまい、デプロイできません。
環境は
os mac Big sur
ruby '2.6.3'

Gemfile.lockの最後の文が
BUNDLED WITH
2.1.4
となっているので、bundleのバージョンは2.1.4になってると思うのですが‥‥
エラー文

remote:  !
remote:  !     Could not detect rake tasks
remote:  !     ensure you can run `$ bundle exec rake -P` against your app
remote:  !     and using the production group of your Gemfile.
remote:  !     Activating bundler (2.1.4) failed:
remote:  !     Could not find 'bundler' (2.1.4) required by your /tmp/build_7b10b0f3/Gemfile.lock.
remote:  !     To update to the latest version installed on your system, run `bundle update --bundler`.
remote:  !     To install the missing version, run `gem install bundler:2.1.4`
remote:  !     Checked in 'GEM_PATH=/tmp/build_7b10b0f3/vendor/bundle/ruby/2.6.0', execute `gem env` for more information
remote:  !
remote:  !     To install the version of bundler this project requires, run `gem install bundler -v '2.1.4'`
remote:  !

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', '>= 6.0.3.4'
# Use sqlite3 as the database for Active Record
gem 'pg',             '0.17.1'
# Use Puma as the app server
gem 'puma', '~> 4.1'
# Use SCSS for stylesheets
gem 'sass-rails', '>= 6'
# Transpile app-like JavaScript. Read more: https://github.com/rails/webpacker
gem 'webpacker', '~> 4.0'

# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.7'
# 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', '>= 1.4.2', require: false

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', 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', '>= 3.3.0'
  gem 'listen', '~> 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :test do
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '>= 2.15'
  gem 'selenium-webdriver'
  # Easy installation and use of web drivers to run system tests with browsers
  gem 'webdrivers'
end

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

gem 'devise'
gem "refile", require: "refile/rails", github: 'manfe/refile'
gem "refile-mini_magick"
gem "bulma-rails"

こいつと１週間近く格闘しています。
herokuでデプロイできないのであれば、他の方法を考えるのですが、おすすめはありますか？