3.x: Fix toFlowable(ERROR) not cancelling on MBE

[akarnokd]

There was a missing upstream.cancel() for when the overflow happens.

Fixes #7081

[codecov]

Codecov Report

Merging #7083 into 3.x will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7083      +/-   ##
============================================
+ Coverage     99.51%   99.54%   +0.02%     
  Complexity     6670     6670              
============================================
  Files           742      742              
  Lines         47275    47276       +1     
  Branches       6374     6374              
============================================
+ Hits          47045    47060      +15     
+ Misses          105      100       -5     
+ Partials        125      116       -9     

Impacted Files	Coverage Δ	Complexity Δ
...perators/flowable/FlowableOnBackpressureError.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0.00%> (-1.39%)	57.00% <0.00%> (-1.00%)
...rnal/operators/observable/ObservableObserveOn.java	98.69% <0.00%> (-1.31%)	3.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	97.73% <0.00%> (-1.30%)	6.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	96.89% <0.00%> (-1.04%)	2.00% <0.00%> (ø%)
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	6.00% <0.00%> (ø%)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0.00%> (-0.91%)	2.00% <0.00%> (ø%)
...al/operators/observable/ObservableWindowTimed.java	97.09% <0.00%> (+0.32%)	4.00% <0.00%> (ø%)
...operators/flowable/FlowableConcatMapScheduler.java	99.60% <0.00%> (+0.39%)	4.00% <0.00%> (ø%)
... and 9 more

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[vanniktech]

Also here, does Observable behave correctly?