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

- Allow use of older versions of TLS on Bookworm for testing of
related optional parameters.
- Install the IMAP CLI testing tool under its own virtual environment
in order to avoid warnings and errors when trying to install it
globally under Debian 12 Bookworm.
- Update the lists of expected TLS protocols and ciphers.

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

- Shorten the backup client username so it would be under 32
characters.
- Switch all helper VMs to using the Debian 11 Bullseye as well.
- Drop the architecture suffix from hostnames.
- Update the hostname for client VMs.
- Fix the incorrect format for the message ID used in various mail
delivery tests.

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

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Use Debian Buster for helper machines.
- Drop Stretch-specific code and tests.

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

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Improve handling of configured IP in tests to avoid hard-coding the
value in the relevant test for Postfix configuration file content.

MAR-150: Use fixtures for X.509 artefacts in the mail_server role:

- Removed the statically generated artefacts.
- Generate X.509 artefacts for tests using Gimmecert.
- Updated paths to point to generated artefacts.
- Introduced cleanup playbook for removing generated artefacts.
- Increase allocated RAM for the mail server to avoid OOM and
swapping.

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

- Updated Molecule configuration to include set-up of additional
instances for testing.
- Updated configuration for test instances.
- Use separate clients in testing of Jessie/Stretch instances.
- Duplicate private keys/certificates for testing of mandatory
parameters on Debian 9.
- Refactored testing of mail deliveries (via swaks) to use
test-generated message ID - improves reliability and solves some
incompatibilities between swaks version in Jessie and Stretch.
- Updated tests for TLS testing to take into account newer OpenSSL
error/output messages. A bit of an ugly hack at the moment, but
beats duplicating tests for now.

MAR-132: Fixed creation of test LDAP entries mail_server role tests:

- Do not use the ldap_entries option of the ldap_server role.
- Invoke the ldap_entry and ldap_attr modules directly.

MAR-129: Updated mail_server role for new Molecule and Ansible:

- Moved variables from test playbooks into group/host vars.
- Fixed linting errors.
- Reference custom top-level linting configuration file in tests.
- Fixed deprecation errors reported by Ansible.
- Updated how the hosts on which the tests should be run are
referenced.