Ubuntu repository migration: Provide cleanup documentation for the old repository

[rloutrel]

Describe the bug
The repository for ubuntu based OSes hase migrated from launchpad to Mozilla.
In order to help users to keep a clean system, you might add in your readme how to remove the previous repository.

VPN version:
2.36

Affected Platforms:
Ubuntu

Prerequisites:
Have installed mozilla VPN as deb package before version 2.36 (so 2.35 and before)

Steps to reproduce:

Check for the old repository:

computer:~$ add-apt-repository --list | grep URIs
URIs: http://de.archive.ubuntu.com/ubuntu/
URIs: http://security.ubuntu.com/ubuntu
URIs: https://ppa.launchpadcontent.net/libreoffice/libreoffice-still/ubuntu/
URIs: https://esm.ubuntu.com/infra/ubuntu
URIs: https://esm.ubuntu.com/apps/ubuntu
URIs: https://ppa.launchpadcontent.net/kelebek333/mint-tools/ubuntu/

Check for the old key

computer:~$ apt-key list
/etc/apt/trusted.gpg.d/libreoffice-ubuntu-ppa.gpg
-------------------------------------------------
pub   rsa4096 2024-05-02 [SC]
      CDDE 43BA 4DE3 F09D 7CE7  0166 73AE E920 AD55 F5A0
uid           [ unknown] Launchpad PPA for LibreOffice Packaging

/etc/apt/trusted.gpg.d/mozillacorp-ubuntu-mozillavpn.gpg
--------------------------------------------------------
pub   rsa4096 2020-12-14 [SC]
      2F8D 6250 9C85 5C0D 2095  4184 1E51 5603 13C4 44E0
uid           [ unknown] Launchpad PPA for Mozilla

/etc/apt/trusted.gpg.d/solaar-unifying-ubuntu-stable.gpg
--------------------------------------------------------
pub   rsa4096 2018-12-03 [SC]
      643F F34A F915 1DC8 14DD  3A1B 2303 D595 EE90 7473
uid           [ unknown] Launchpad PPA for Solaar

Expected result:

Repository:

if https://ppa.launchpadcontent.net/mozillacorp/mozillavpn/ubuntu is in the list, the old repository is there. Otherwise, every thing OK (check for the old key)

Key:

if a key looking like the following exist (focus on uid value), then the old key is to be removed (save the long string looking like 2F8D 6250 9C85 5C0D 2095 4184 1E51 5603 13C4 44E0 here). Otherwise, every thing is OK for the key (check for the repository):

/etc/apt/trusted.gpg.d/mozillacorp-ubuntu-mozillavpn.gpg
--------------------------------------------------------
pub   rsa4096 2020-12-14 [SC]
      2F8D 6250 9C85 5C0D 2095  4184 1E51 5603 13C4 44E0
uid           [ unknown] Launchpad PPA for Mozilla

Actual result:
https://ppa.launchpadcontent.net/mozillacorp/mozillavpn/ubuntu is in the list (and the )

Additional notes:
Actions to do:

# Remove the repository 
sudo add-apt-repository --remove ppa:mozillacorp/mozillavpn

# remove the key (replace key ID with yours)
sudo apt-key del 2F8D 6250 9C85 5C0D 2095  4184 1E51 5603 13C4 44E0

┆Issue is synchronized with this Jira Task