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

- Introduced internal parameters for controlling differing package
names, service names, and paths for PHP FPM package.
- Added Debian 9 machines to Molecule configuration, including the
client machine.
- Restructured slightly preparaiton playbook to support both Jessie
and Stretch.
- Added custom pytest fixture for having a better way to determine
expected package names etc related to PHP.
- Created copy of private key/certificate pair used for testing of
mandatory parameters (to be used with Stretch machine).
- Fixed invalid specification for hosts on top of which the
connectivity test should be run.
- Updated a couple of task names (avoiding to reference PHP 5).
- Updated documentation.