Refactor the code so we don't use the Observable class

[Simon97]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

In the api_client, we are using the Observable class from the rxdart library but it is not a part of the newest version. The task is to refractor the code and upgrade the dependencies in order to block further use of deprecated classes.

Describe the solution you'd like A clear and concise description of what you want to happen. A solution is described at the following link: https://pub.dev/packages/rxdart#-changelog-tab-

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered. Change the code manually?

Additional context Add any other context or screenshots about the feature request here.

The same problem exists in the weekplanner repo: https://github.com/aau-giraf/weekplanner/issues/499

[jkrod]

Info:

Synes ikke jeg kunne se andet end en changelog for RX Dart librariet i det link. Men har fundet et script der vidst kan køres via en flutter command for at opdatere koden. Check evt. lige selv, hvis man arbejder på dette issue.

FRA: https://stackoverflow.com/questions/59336428/observable-is-deprecated-in-rxdart-0-23-1