ubuntuへのHeroku CLIの導入ができない[apt-get update時のエラー？]

前提・実現したいこと

[前提]
railsで作成中のアプリケーションにHeroku CLIを導入しようとした際にエラーが発生しました。

[実現したいこと]
エラーを解決してHeroku CLIを導入したいです。

発生している問題・エラーメッセージ

ubuntu
1$ curl https://cli-assets.heroku.com/install-ubuntu.sh | sh
2  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
3                                 Dload  Upload   Total   Spent    Left  Speed
4100  1232  100  1232    0     0   3767      0 --:--:-- --:--:-- --:--:--  3767
5This script requires superuser access to install apt packages.
6You will be prompted for your password by sudo.
7[sudo] password for boko: 
8+ dpkg -s apt-transport-https
9+ echo 
10+ apt-get update
11Hit:1 https://dl.yarnpkg.com/debian stable InRelease
12Get:2 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]                                
13Get:3 http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease [81.7 kB]                             
14Hit:4 http://archive.ubuntu.com/ubuntu focal InRelease                                                   
15Hit:5 http://archive.ubuntu.com/ubuntu focal-updates InRelease                             
16Get:6 http://archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
17Err:3 http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease
18  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FCC7D46ACCC4CF8
19Reading package lists... Done
20W: GPG error: http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FCC7D46ACCC4CF8
21E: The repository 'http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease' is not signed.
22N: Updating from such a repository can't be done securely, and is therefore disabled by default.
23N: See apt-secure(8) manpage for repository creation and user configuration details.
24
25$ heroku --version
26heroku: command not found

下記のインストール方法はsnapがサポートされていないので上記のようなインストール方法を試みました。

ubuntu
1$ sudo snap install --classic heroku
2[sudo] password for boko: 
3Interacting with snapd is not yet supported on Windows Subsystem for Linux.
4This command has been left available for documentation purposes only.

試したこと

公式サイトのインストール方法紹介
↑こちらの方法でのインストールを試みました。

インストールの際に参考にしたQiita記事

UbuntuでGPGエラーが出た時の対処法
↑こちらの記事を試してみましたが以下のようにうまくいきませんでした。

ubuntu
1$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys
2[sudo] password for boko: 
3Executing: /tmp/apt-key-gpghome.j41YUaXCBy/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys
4
5$ curl https://cli-assets.heroku.com/install-ubuntu.sh | sh
6  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
7                                 Dload  Upload   Total   Spent    Left  Speed
8100  1232  100  1232    0     0   3666      0 --:--:-- --:--:-- --:--:--  3666
9This script requires superuser access to install apt packages.
10You will be prompted for your password by sudo.
11[sudo] password for boko: 
12+ dpkg -s apt-transport-https
13+ echo 
14+ apt-get update
15Hit:1 https://dl.yarnpkg.com/debian stable InRelease
16Get:2 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
17Hit:3 http://archive.ubuntu.com/ubuntu focal InRelease                                                   
18Hit:4 http://archive.ubuntu.com/ubuntu focal-updates InRelease                                           
19Get:5 http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease [81.7 kB]
20Get:6 http://archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
21Err:5 http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease    
22  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FCC7D46ACCC4CF8
23Reading package lists... Done
24W: GPG error: http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FCC7D46ACCC4CF8
25E: The repository 'http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease' is not signed.
26N: Updating from such a repository can't be done securely, and is therefore disabled by default.
27N: See apt-secure(8) manpage for repository creation and user configuration details.
28
29$ heroku --version
30heroku: command not found