Update Molecule to remove Alpine Linux 3.15

[alessfg]

Proposed changes

Update Molecule to remove Alpine Linux 3.15 and automate Molecule downgrade scenario version checks.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the CONTRIBUTING document.
• I have added Molecule tests that prove my fix is effective or that my feature works.
• I have checked that any relevant Molecule tests pass after adding my changes.
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md).