The official deb.mariadb.org mirrors are intended for distribution of the
current MariaDB releases. When a version goes end-of-life, they are
removed from those mirrors.

The upgrade tests should however work even after EOL. While we do want
users to stop using EOL versions, we still expect the newer versions to
support upgrades from old versions to the current versions. Therefore we
should continue testing upgrades from EOL versions, and for that to work,
switch the CI to use the archive.mariadb.org repositories instead.

MERGE NOTE: This commit was made on the oldest branch with the salsa-ci.yml
file. When merging 10.5->10.6->...->10.12 please include this commit in
the merge and ensure all files end up with the change:

    deb.mariadb.org/10.([0-9]+)/ -> archive.mariadb.org/mariadb-10.$1/repo/