patch_platform: support downgrade/rollback via allow_downgrade flag

[ZackStr]

Feature Request

Summary

The patch_platform playbook currently only supports upgrading Itential Platform
to a newer version. There is no supported path to roll back to a previous RPM
version using the same tooling.

Problem

When a patch upgrade causes unexpected issues in a production or staging
environment, operators need a fast, reliable rollback path. Today, the
patch_platform collection cannot be used for this purpose because the
underlying Ansible dnf module will treat a lower-version RPM as already
satisfied and take no action — it silently reports ok without downgrading.

Customers are forced to manually intervene with raw rpm or dnf commands
outside of the deployer, which defeats the purpose of having a standardized
deployment tool and breaks auditability.

Proposed Solution

Add an allow_downgrade flag (defaulting to false) to the patch_platform
role. When set to true, the role should pass allow_downgrade: true to the
Ansible dnf module, enabling operators to install an older RPM version over
a newer one.

Example inventory usage:
platform_allow_downgrade: true

Acceptance Criteria

• patch_platform succeeds when platform_packages points to an older RPM
  than what is currently installed, and platform_allow_downgrade: true is set
• Default behavior (upgrade only) is unchanged when the flag is omitted or false
• Documentation updated in patch_itential_platform_guide.md