Due to Python 3 being the default in all supported OSs, we may remove the logic that dealt with
detecting the OSs Python version and consequently installed a different set of packages.

- removed dependency on Python 2 packages (python-psycopg2)
- rephrased named steps in CI as it is explicitely set in `python-version`
- require Ansible 2.12 as it supports the auto-detection of the OS Python interpreter
- removed explicit setting of Python version in variables, playbooks and inventory
- removed unsupported configurations from test inventory

Ansible 10 uses ansible-core 2.17+ which drops Python 3.6 support and thus makes it incompatible
with Rocky Linux 8 or other RHEL 8 derivatives.