MAR-239: Dropped support for Debian 11 Bullseye from the backup_client role:

- Switch to using Paramiko instead of pexpect backend (therefore
avoiding using the external SSH client binary).

MAR-198: Implement removal of expired backups via backup_client role:

- In addition to expired backup removals, ensure that the backup
does not proceed if previous execution step has failed. This means
that pre-scripts now have to run without errors for the main backup
to kick-in.
- Could not use full _and_ chaining of duply. For example, if we have
A_and_B_and_C (where A/B/C are duply backup commands), if A
fails, B will be skipped, but C will still run because B has ran
without_errors.

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-149: Switch to using File.content_string instead of File.content in tests for all roles:

- The .content variant returns contents as bytes, while the
content_string returns a string (which is what is wanted in
practically all cases).

MAR-129: Updated backup_client role tests:

- Update how the hosts are being referenced in tests (for limiting the
test runs).
- Use "state: present" for package presence/installation.
- Break-up long lines in tasks so they are under 160 characters
length.
- Fixed YAML linting issues.

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.