Herokuでデプロイできない

前提・実現したいこと

railsアプリケーションを作成しており、Herokuでデプロイしようとするとエラーが出ました。
エラーを解消し、デプロイできるようにしたいです。

発生している問題・エラーメッセージ

$ git push heroku master
　　　　　　　　　　　　　　＜省略＞
remote:  !     Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
remote: 			Detected buildpacks: Ruby,Node.js
remote: 			See https://devcenter.heroku.com/articles/buildpacks#buildpack-detect-order
remote: -----> Ruby app detected
remote: -----> Installing bundler 2.1.4
remote: -----> Removing BUNDLED WITH version in the Gemfile.lock
remote: -----> Compiling Ruby/Rails
remote:        Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-20/ruby-2.6.5.tgz -s -o - | tar zxf - ' failed on attempt 1 of 3.
remote:        Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-20/ruby-2.6.5.tgz -s -o - | tar zxf - ' failed on attempt 2 of 3.
remote: 
remote:  !
remote:  !     The Ruby version you are trying to install does not exist on this stack.
remote:  !     
remote:  !     You are trying to install ruby-2.6.5 on heroku-20.
remote:  !     
remote:  !     Ruby ruby-2.6.5 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:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed
remote:  !
remote:  ! ## Warning - The same version of this code has already been built: a63ec93e7648323a70388b9e8f3c43c88777f899
remote:  !
remote:  ! We have detected that you have triggered a build from source code with version a63ec93e7648323a70388b9e8f3c43c88777f899
remote:  ! at least twice. One common cause of this behavior is attempting to deploy code from a different branch.
remote:  !
remote:  ! If you are developing on a branch and deploying via git you must run
remote:  !
remote:  !     git push heroku <branchname>:main
remote:  !
remote:  ! This article goes into details on the behavior:
remote:  !   https://devcenter.heroku.com/articles/duplicate-build-version
remote: 
remote: Verifying deploy...
remote: 
remote: !	Push rejected to cafe-log.
remote: 
To https://git.heroku.com/cafe-log.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/cafe-log.git'

該当のソースコード

ソースコード

試したこと

$ heroku stack:set heroku-16
上記のコマンドを実行し再度、herokuへpushしました。

発生している問題・エラーメッセージ

git push heroku master    
　　　　　　　　　　　＜省略＞
remote: -----> Building on the Heroku-16 stack
remote:  !     Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
remote: 			Detected buildpacks: Ruby,Node.js
remote: 			See https://devcenter.heroku.com/articles/buildpacks#buildpack-detect-order
　　　　　　　　　　＜省略＞
remote:   _                    _                __   __    ______ ____  _
remote:  | |                  | |              /_ | / /   |  ____/ __ \| |
remote:  | |__   ___ _ __ ___ | | ___   _ ______| |/ /_   | |__ | |  | | |
remote:  | '_ \ / _ \ '__/ _ \| |/ / | | |______| | '_ \  |  __|| |  | | |
remote:  | | | |  __/ | | (_) |   <| |_| |      | | (_) | | |___| |__| | |____
remote:  |_| |_|\___|_|  \___/|_|\_\__,_|      |_|\___/  |______\____/|______|
remote: 
remote: This app is using the Heroku-16 stack, which is deprecated:
remote: https://devcenter.heroku.com/changelog-items/1958
remote: 
remote: From May 1st 2021, Heroku-16 will be end-of-life, and apps using it
remote: will no longer receive security updates, and be run at your own risk.
remote: 
remote: From June 1st 2021, builds will no longer be allowed for Heroku-16 apps
remote: 
remote: Please upgrade to a newer stack as soon as possible:
remote: https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack
remote: https://help.heroku.com/0S5P41DC/heroku-16-end-of-life-faq
remote: 
remote:  !
remote:  ! ## Warning - The same version of this code has already been built: a63ec93e7648323a70388b9e8f3c43c88777f899
remote:  !
remote:  ! We have detected that you have triggered a build from source code with version a63ec93e7648323a70388b9e8f3c43c88777f899
remote:  ! at least twice. One common cause of this behavior is attempting to deploy code from a different branch.
remote:  !
remote:  ! If you are developing on a branch and deploying via git you must run
remote:  !
remote:  !     git push heroku <branchname>:main
remote:  !
remote:  ! This article goes into details on the behavior:
remote:  !   https://devcenter.heroku.com/articles/duplicate-build-version
remote: 
remote: Verifying deploy... done.
To https://git.heroku.com/cafe-log.git
 * [new branch]      master -> master

このような状態になっており、対処方法がわかりません。
ご教授のほどよろしくお願いします。