docs: older version of Ansible needed for RHEL 8

[richardlau]

Update our Ansible documentation to note that newer versions of Ansible are not compatible with RHEL 8 and provide the last known back-level compatible versions of ansible and ansible-core that work.

---

With the latest Ansible, I was getting this error when running against our RHEL 8 hosts:

TASK [package-upgrade : upgrade installed packages] *************************************************************************************************************************************************************
fatal: [release-osuosl-rhel8-arm64-1]: FAILED! => {"changed": false, "msg": "Could not import the dnf python module using /usr/bin/python3.12 (3.12.11 (main, Jun 19 2025, 15:40:50) [GCC 8.5.0 20210514 (Red Hat 8.5.0-27)]). Please install `python3-dnf` package or ensure you have specified the correct ansible_python_interpreter. (attempted ['/usr/libexec/platform-python', '/usr/bin/python3', '/usr/bin/python'])", "results": []}

python3-dnf is required for Ansible, but on RHEL 8 is only provided for the system python (/usr/libexec/platform-python, Python 3.6). Attempting to set the Python interpreter to the system Python (as opposed to 3.12) fails, and this appears to be known/deliberate:
Refs: https://forum.ansible.com/t/almalinux-and-python3-dnf/44262/4
Refs: https://forum.ansible.com/t/python-3-7-impact-on-el8-future-for-el9/6229/13