3.x: API promotions for 3.1.0

[akarnokd]

Promote the following methods to standard:

• Flowable.onBackpressureReduce() + 1
• RxJavaPlugins.getOnParallelSubscribe() and RxJavaPlugins.setOnParallelSubscribe()
• TestScheduler([...] boolean useOnScheduleHook)

[codecov]

Codecov Report

Merging #7296 (e017149) into 3.x (3330943) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7296      +/-   ##
============================================
+ Coverage     99.51%   99.53%   +0.02%     
- Complexity     6740     6744       +4     
============================================
  Files           747      747              
  Lines         47347    47347              
  Branches       6376     6376              
============================================
+ Hits          47116    47129      +13     
+ Misses          107      100       -7     
+ Partials        124      118       -6     

Impacted Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
...ava3/internal/schedulers/SchedulerPoolFactory.java	100.00% <ø> (ø)
...va/io/reactivex/rxjava3/plugins/RxJavaPlugins.java	100.00% <ø> (ø)
...io/reactivex/rxjava3/schedulers/TestScheduler.java	100.00% <ø> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	95.85% <0.00%> (-2.08%)	⬇️
...vable/ObservableFlatMapCompletableCompletable.java	98.48% <0.00%> (-1.52%)	⬇️
...rnal/operators/flowable/FlowableSequenceEqual.java	98.78% <0.00%> (-1.22%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	96.82% <0.00%> (-0.80%)	⬇️
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	⬇️
...ternal/operators/observable/ObservableFlatMap.java	97.51% <0.00%> (-0.36%)	⬇️
... and 8 more

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[MelnikovAG]

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