我收到“存储库 focal-security Release”不再包含 Ubuntu 20.04 的 Release 文件

Question

今天我的系统出现了这个错误。当我在系统上工作时，顶部栏出现了一个红色通知，我试图查看哪里出了问题。

在终端中我尝试使用以下命令：

sudo apt update

此后，它产生了这个错误：

❯ sudo apt update
Alias: _ apt update
[sudo] password for user: 
Ign:1 http://security.ubuntu.com/ubuntu focal-security InRelease                                                                                     
Err:2 http://security.ubuntu.com/ubuntu focal-security Release                                                                                       
  Cannot initiate the connection to security.ubuntu.com:80 (2001:67c:1562::18). - connect (101: Network is unreachable) Cannot initiate the connection to security.ubuntu.com:80 (2001:67c:1360:8001::23). - connect (101: Network is unreachable) Cannot initiate the connection to security.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) Cannot initiate the connection to security.ubuntu.com:80 (2001:67c:1360:8001::24). - connect (101: Network is unreachable)
...                             
Reading package lists... Done                                                                                                                        
E: The repository 'http://security.ubuntu.com/ubuntu focal-security Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

我该如何解决这个安全问题？