Add gpg key for mirroring newer Ubuntu releases: 2018-signing-key

[Mark Cunningham]

This is a request to add the newer (2018) Ubuntu Archive signing key to Landscape.

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pub rsa4096 2018-09-17 [SC]
      F6EC B376 2474 EDA9 D21B 7022 8719 20D1 991B C93C
uid [ unknown] Ubuntu Archive Automatic Signing Key (2018) <email address hidden>
---

Currently this needs to be done manually, in order to mirror and sync the Jammy 22.04 repository.

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sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 871920D1991BC93C

sudo apt-key export 871920D1991BC93C > 2018-signing-key.pem

landscape-api import-gpg-key 2018-signing-key 2018-signing-key.pem
---

Adding this would simplify mirroring newer releases of Ubuntu using Landscape.

[Simon Poirier (simpoir)]

Chiming in to say we probably want to continue shipping obsolete keys in
canonical/landscape/model/repository/constants.py since a landscape server might be mirror past suites just as much as future suites.

Side note that apt-key is deprecated, but the key can be dumped from system keyrings
gpg --keyring /usr/share/keyrings/ubuntu-archive-keyring.gpg --export --export -a F6ECB3762474EDA9D21B7022871920D1991BC93C

[Ania Quen (aniaqueen232)]

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