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

- Drop the orphan config file for backup server helper.

MAR-191: Drop Debian 10 Buster support for the backup_client role:

- Internal parameter for Python's pexpect package is no longer
required - newer versions of Debian have fully switche to using the
Python 3 version of it for duplicity.
- Fix prepare playbook task to iterate over list of backup users
instead of hard-coding the values.
- Drop the directory set-up task that seems to serve no purpose (some
leftover from previous refactoring most likely.
- Update supported version information in docs and metadata.

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

- Duplicity on Buster and Bullseye uses different Python version (2 vs
3), install the correct Python pexpect package.
- Switch the helper backup server to using Bullseye, and tweak its
SSHD configuration accordingly.

MAR-189: Refactor hostname pattern for backup_client role (buster/bullseye/bookworm):

- Multiple releases one after the other have names that begin with
letter b, making it necessary to use more than just one letter.
- Use full distro codename, but shorten the parameters to param in
order to avoid issues related to 32-character username limits (when
bak-HOSTNAME user gets created).
- Drop some leftover references to Debian Stretch.
- Drop the arch from hostnames - default is to use 64-bit boxes
anyway.

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

- Updated tests.
- Refactored one of the test to determine hostname dynamically.
- Update role reference documentation.

MAR-129: Updated Molecule configuration for the backup_client role:

- Updates necessary for use with Molecule version 2.14.
- Reference custom yamllint configuration file with custom rules.
- Fix YAML linting errors in Molecule's own files.

MAR-128: Upgraded tests for backup_client 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).
- Updated backup server SSH deployment to have chroot the users for a
bit easier set-up, and updated relevants playbooks/tests for this
purpose.