Update Molecule to support NGINX Plus R31

[alessfg]

Proposed changes

• Add Alpine Linux 3.19 to the list of NGINX Plus tested and supported distributions.
• Remove Alpine Linux 3.15 from the list of NGINX Plus tested and supported distributions.

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).