[BUG][Release-Blocker] TUM Online Login Flow broken

[kordianbruck]

1. enter your TUM ID
2. Click next
3. App crashes

This is on the master branch.

FATAL EXCEPTION: main
Process: de.tum.in.tumcampus, PID: 15030
java.lang.IllegalArgumentException: Unable to create call adapter for class io.reactivex.Single
    for method TUMOnlineAPIService.requestToken
	at retrofit2.Utils.methodError(Utils.java:54)
	at retrofit2.HttpServiceMethod.createCallAdapter(HttpServiceMethod.java:116)
	at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:67)
	at retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:39)
	at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:202)
	at retrofit2.Retrofit$1.invoke(Retrofit.java:160)
	at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
	at $Proxy5.requestToken(Unknown Source)
	at de.tum.in.tumcampusapp.api.tumonline.TUMOnlineClient.requestToken(TUMOnlineClient.kt:96)
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.requestNewToken(OnboardingStartFragment.kt:137)
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.onNextPressed(OnboardingStartFragment.kt:124)
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.onViewCreated$lambda$3$lambda$2(OnboardingStartFragment.kt:90)
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.$r8$lambda$jaDRk2WVYbClKJtskr88jyi9XT4(OnboardingStartFragment.kt:0)
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment$$ExternalSyntheticLambda3.onClick(R8$$SyntheticClass:0)
	at android.view.View.performClick(View.java:7520)
	at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1213)
	at android.view.View.performClickInternal(View.java:7489)
	at android.view.View.access$3600(View.java:826)
	at android.view.View$PerformClick.run(View.java:28555)
	at android.os.Handler.handleCallback(Handler.java:938)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loop(Looper.java:233)
	at android.app.ActivityThread.main(ActivityThread.java:8068)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:631)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:978)
Caused by: java.lang.IllegalStateException: Single return type must be parameterized as Single<Foo> or Single<? extends Foo>
	at retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory.get(RxJava2CallAdapterFactory.java:118)
	at retrofit2.Retrofit.nextCallAdapter(Retrofit.java:253)
	at retrofit2.Retrofit.callAdapter(Retrofit.java:237)
	at retrofit2.HttpServiceMethod.createCallAdapter(HttpServiceMethod.java:114)
	at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:67) 
	at retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:39) 
	at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:202) 
	at retrofit2.Retrofit$1.invoke(Retrofit.java:160) 
	at java.lang.reflect.Proxy.invoke(Proxy.java:1006) 
	at $Proxy5.requestToken(Unknown Source) 
	at de.tum.in.tumcampusapp.api.tumonline.TUMOnlineClient.requestToken(TUMOnlineClient.kt:96) 
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.requestNewToken(OnboardingStartFragment.kt:137) 
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.onNextPressed(OnboardingStartFragment.kt:124) 
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.onViewCreated$lambda$3$lambda$2(OnboardingStartFragment.kt:90) 
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment.$r8$lambda$jaDRk2WVYbClKJtskr88jyi9XT4(OnboardingStartFragment.kt:0) 
	at de.tum.in.tumcampusapp.component.ui.onboarding.OnboardingStartFragment$$ExternalSyntheticLambda3.onClick(R8$$SyntheticClass:0) 
	at android.view.View.performClick(View.java:7520) 
	at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1213) 
	at android.view.View.performClickInternal(View.java:7489) 
	at android.view.View.access$3600(View.java:826) 
	at android.view.View$PerformClick.run(View.java:28555) 
	at android.os.Handler.handleCallback(Handler.java:938) 
	at android.os.Handler.dispatchMessage(Handler.java:99) 
	at android.os.Looper.loop(Looper.java:233) 
	at android.app.ActivityThread.main(ActivityThread.java:8068) 
	at java.lang.reflect.Method.invoke(Native Method) 
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:631) 
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:978) 
2023-12-02 21:13:15.467 15030-30301 sqlite3_android         de.tum.in.tumcampus                  E  [IKR-38846] ONEPLUS_NAME_PARTS_MATCH SQLITE_OK
---------------------------- PROCESS ENDED (15030) for package de.tum.in.tumcampus ----------------------------