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

- Some tweaks had to be done to test the TLS, but it would be useful
to revisit the TLS configuration in general in the future, and to
modernise it for TLSv1.2 and TLSv1.3, especially once the supports
for Debian 11 Bullseye is dropped.

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

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Drop Stretch-specific code and tests.

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

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Refactor the code around handling of different directories and files
for PHP 7.0 (Debian Stretch) and PHP 7.3 (Debian Buster).
- Separate socket directory tests for WSGI and PHP applications (due
to differences in paths for PHP in Debian Stretch and Debian
Buster).

MAR-128: Upgraded tests for ROLE_NAME 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 hostname instead of hard-coding it in a
couple of tests.
- Renamed hosts instances to include Debian version in their
hostnames.
- Fixed some linting issues.