Flask-Restplus depreciated, move to Flask-RestX

[larsbotics]

Is your feature request related to a problem? Please describe.
As I've went through the project and tried to analyze the structure of it, I came by the Flask-Restplus repository (which this project is based on). In their README, they write that:
This project has been forked to Flask-RESTX and will be maintained by by the python-restx organization. Flask-RESTPlus should be considered unmaintained.

Describe the solution you'd like
To guarantee that future releases of LOST are always up to date, it might be a good idea to move to Flask-RestX.
Apparently, the only thing that needs to be changed is the import source (but haven't done deeper research).

Links:
https://github.com/noirbizarre/flask-restplus
https://github.com/python-restx/flask-restx

[gereonreus]

Adapted on branch 2.x