feat: update default_postfix to optional_postfix

[sdeguchi]

Overview/Summary

Update starter module default_postfix to optional_postfix as there is confusion between the default_postfix and postfix_number. Want to update to remove the confusion between the two.

This PR fixes/adds/changes/removes

1. Update default_postfix to optional_postfix

Breaking Changes

1. Breaks previous versions of FSI/SLZ starter module using default_postfix

Testing Evidence

In progress...

As part of this Pull Request I have

• Checked for duplicate Pull Requests
• Associated it with relevant issues, for tracking and closure.
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated relevant and associated documentation.

[jaredfholgate]

LGTM