Fix bug where HTTP requests were incorrectly cancelled

[FelixSFD]

hopefully fixes #167

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First, I changed the HTTP call from .data() to .dataTask() with a completion handler. I did this, because I couldn't find out, which exact line was throwing the exception, when the request was cancelled. Because I didn't want to change the rest of the class, I wrapped this new call inside withCheckedThrowingContinuation() to turn this into an async call. This didn't fix the issue completely, because there was a new warning in the log:

SWIFT TASK CONTINUATION MISUSE [...] leaked its continuation!

This error told me, what happened to the running tasks when navigating quickly: Something is wrong with the memory management of this class. So I switched the property grocyVM in the views from @SteteObject to @ObservedObject, which is recommended for properties that are not created inside the view, but injected instead: https://www.avanderlee.com/swiftui/stateobject-observedobject-differences/#should-i-use-stateobject-for-all-views-using-the-same-instance