MAR-192: Added support for Debian 12 Bookworm to backup_server role:

- Use test parametrisation instead of looping over a list when testing
correct key usage.
- Replace deprecated key algorithm (in Debian 12 Bookworm) for test
purposes (it just needs to be one of the RSA variants).

MAR-189: Added support for Debian 11 Bullseye to backup_server role:

- Drop the architecture version from the test machine names.

MAR-181: Drop support for Debian 9 Stretch from the backup_server role:

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.

MAR-151: Added support for Debian 10 Buster to backup_server role:

- Updated role reference documentation.
- Updated role meta information.
- Updated tests.
- Do not use distribution version-specific SSH configuration file for
backup server SSH daemon.

MAR-152: Drop a couple of references to Debian 8 Jessie from multiple roles:

- Mark workaround as TODO entry (so maybe it can get fixed at some
point.
- Drop SSH known hosts entries in tests for backup_server role.
- Fix some erroneous comments in tests for mail_server
role (references to Debian 8 Jessie).
- Drop reference to Debian 8 Jessie from the preseed file template.

MAR-132: Added support for Debian 9 (Stretch) to backup_server role:

- Deploy slightly different sshd_config file for Stretch (DSA key
unused).
- Updated Molecule tests to cover testing on Debian 9.
- Updated role reference documentation.

MAR-128: Upgraded tests for backup_server role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (eases
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Switched to extracting IP address instead of hard-coding it in a
couple of tests.
- Moved test for checking available authentication mechanisms for
backup SSH server to be part of testing of parameters_optional only
for now (it was hard coded to that IP, and fails on
parameters-mandatory due to iptables not opening correct ports).