2.x: ProGuard R8 dont warn Flow* classes

[akarnokd]

#6704 didn't work, probably because the filter has to match a filename java.util.concurrent.Flow$Publisher, not some parent class name.

Having -dontwarn java.util.concurrent.Flow* did work in a test project's proguard file locally.

[codecov]

Codecov Report

Merging #6705 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6705      +/-   ##
============================================
- Coverage     98.26%   98.23%   -0.03%     
- Complexity     6347     6349       +2     
============================================
  Files           677      677              
  Lines         45526    45526              
  Branches       6333     6333              
============================================
- Hits          44735    44724      -11     
- Misses          246      254       +8     
- Partials        545      548       +3

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-3.87%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-2.99%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	96.58% <0%> (-1.71%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.53% <0%> (-1.59%)	3% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	96.61% <0%> (-0.97%)	8% <0%> (ø)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0%> (-0.86%)	2% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	99.24% <0%> (-0.76%)	3% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	88.67% <0%> (-0.65%)	3% <0%> (ø)
... and 15 more

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[vanniktech]

Ups. Yes. You need that *

[akarnokd]

This works.