Comment 0 for bug 1798073

[Impact]

 * For LTS releases to be able to bootstrap dual and single signed future releases, and validate all signatures, 2018 archive signing key should be SRUed back

 * Also build process has improved documentation and vague validation that all key snippets are signed correctly

[Test Case]

 * $ apt-key list
...
/etc/apt/trusted.gpg.d/ubuntu-keyring-2018-archive.gpg
------------------------------------------------------
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>
...

apt-key list should contain the 2018 archive key.

[Regression Potential]

 * Build-process, key algo, and key size, and file format are the same as previous key snippets thus supported by all of gpg1 gpg2 gpgv1 gpgv2.

[Other Info]

 * 2018 key is to be used for dual-signing in DD series and up