我正在尝试安装 16.04 的最新更新，以便升级到 18.04。我在使用 phpMyAdmin PPA 时遇到错误，据我了解，这是由于原始包维护者 (nijel) 已删除 PPA 所致。
Ign:20 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial/main amd64 Packages
Ign:13 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial/main all Packages
Ign:14 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial/main Translation-en_US
Ign:21 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial/main Translation-en
Err:16 http://ppa.launchpad.net/phpmyadmin/ppa/ubuntu xenial/main amd64 Packages
404 Not Found [IP: 91.189.95.83 80]
Ign:17 http://ppa.launchpad.net/phpmyadmin/ppa/ubuntu xenial/main all Packages
Ign:18 http://ppa.launchpad.net/phpmyadmin/ppa/ubuntu xenial/main Translation-en_US
Ign:19 http://ppa.launchpad.net/phpmyadmin/ppa/ubuntu xenial/main Translation-en
Err:20 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial/main amd64 Packages
403 Forbidden [IP: 91.189.95.83 80]
Ign:21 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial/main Translation-en
Fetched 2,313 kB in 4s (486 kB/s)
Reading package lists... Done
W: The repository 'http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://ppa.launchpad.net/phpmyadmin/ppa/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/phpmyadmin/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found [IP: 91.189.95.83 80]
E: Failed to fetch http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu/dists/xenial/main/binary-amd64/Packages 403 Forbidden [IP: 91.189.95.83 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
所以我试图删除 PPA：
sudo add-apt-repository --remove ppa:nijel/phpmyadmin
但这不起作用：
Cannot add PPA: 'ppa:~nijel/ubuntu/phpmyadmin'.
The user named '~nijel' has no PPA named 'ubuntu/phpmyadmin'
Please choose from the following available PPAs:
我也试过这个：
sudo add-apt-repository universe
'universe' distribution component is already enabled for all sources.
sudo apt install phpmyadmin
但我得到：
Reading package lists... Done
Building dependency tree
Reading state information... Done
phpmyadmin is already the newest version (4:4.6.6-1+deb.cihar.com~xenial.2).
如何解决这个问题，以便我可以升级到 18.04？