3.x: Fix groupBy not requesting more if a group is cancelled w/ items

[akarnokd]

If a group is cancelled with unconsumed item in its buffer, the operator stopped requesting more thus hanging other groups. In 3.x the operator was somewhat strenghtened for this case except one case when the cancellation happens outside the drain loop.

The fix is to invoke drain() from cancel for it to have the cleanup and replenishment happen.

Fixes #6889

[codecov]

Codecov Report

Merging #6895 into 3.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6895      +/-   ##
============================================
+ Coverage     98.25%   98.28%   +0.02%     
- Complexity     6590     6594       +4     
============================================
  Files           740      740              
  Lines         47293    47294       +1     
  Branches       6430     6430              
============================================
+ Hits          46468    46482      +14     
+ Misses          300      295       -5     
+ Partials        525      517       -8

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/operators/flowable/FlowableGroupBy.java	97.17% <100%> (+1.58%)	3 <0> (ø)	⬇️
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	93.57% <0%> (-4.59%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	89.68% <0%> (-3.18%)	2% <0%> (ø)
...internal/operators/flowable/FlowableFromArray.java	96.61% <0%> (-1.7%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.75% <0%> (-1.57%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...rnal/operators/observable/ObservableObserveOn.java	98.69% <0%> (-1.31%)	3% <0%> (ø)
...ivex/rxjava3/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	87.32% <0%> (-0.71%)	2% <0%> (ø)
... and 21 more

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