feat/adding preppost license to allowed

[germa89]

Description

Add "preppost" license to allowed.

Issue linked

Close #3284

Checklist

• [ ] I have visited and read Develop PyMAPDL.
• [ ] I have tested my changes locally
• [ ] I have added necessary documentation or updated existing documentation.
• [ ] I have followed the PyMAPDL coding style guidelines.
• [ ] I have added appropriate unit tests that also ensure the minimal coverage criteria.
• [x] I have reviewed my changes before submitting this pull request.
• [x] I have linked the issue or issues that are solved to the PR if any.
• [x] I have assigned this PR to myself.
• [x] I have marked this PR as draft if it is not ready to be reviewed yet.
• [x] I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)