3.x: Fix Flowable.flatMap not canceling the inner sources on outer error

[akarnokd]

The outer onError did not cancel the inner sources. The Observable variant works correctly but both received an unit test to verify the behavior.

2.x will be fixed in a separate PR.

Related #6825

[codecov]

Codecov Report

Merging #6826 into 3.x will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6826      +/-   ##
============================================
- Coverage     98.27%   98.26%   -0.01%     
- Complexity     6378     6383       +5     
============================================
  Files           715      715              
  Lines         46482    46485       +3     
  Branches       6371     6373       +2     
============================================
  Hits          45678    45678              
- Misses          285      287       +2     
- Partials        519      520       +1

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/operators/flowable/FlowableFlatMap.java	90.08% <100%> (+0.08%)	4 <0> (ø)	⬇️
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.9% <0%> (-8.26%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-6.34%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	89.68% <0%> (-3.18%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.02% <0%> (-1.17%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
.../operators/observable/ObservableCombineLatest.java	96.17% <0%> (-0.64%)	6% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	89.96% <0%> (-0.34%)	3% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	97.41% <0%> (ø)	6% <0%> (ø)	⬇️
...a3/internal/operators/flowable/FlowableReplay.java	92.81% <0%> (+0.82%)	19% <0%> (ø)	⬇️
... and 10 more

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