3.x: Add onSubscribe hook to ParallelFlowable operators

[akarnokd]

Parallel operators were missing an RxJavaPlugins.onSubscribe hook.

Resolves #7190

[codecov]

Codecov Report

Merging #7191 (5e33ef0) into 3.x (abecb9a) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7191      +/-   ##
============================================
- Coverage     99.58%   99.53%   -0.05%     
- Complexity     6754     6756       +2     
============================================
  Files           747      747              
  Lines         47350    47374      +24     
  Branches       6367     6382      +15     
============================================
+ Hits          47153    47155       +2     
- Misses           91      101      +10     
- Partials        106      118      +12     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/reactivex/rxjava3/core/Scheduler.java	100.00% <100.00%> (ø)	16.00 <2.00> (ø)
...3/internal/operators/parallel/ParallelCollect.java	100.00% <100.00%> (ø)	7.00 <3.00> (ø)
...internal/operators/parallel/ParallelConcatMap.java	100.00% <100.00%> (ø)	5.00 <3.00> (ø)
...ternal/operators/parallel/ParallelDoOnNextTry.java	100.00% <100.00%> (ø)	6.00 <4.00> (ø)
...a3/internal/operators/parallel/ParallelFilter.java	100.00% <100.00%> (ø)	6.00 <4.00> (ø)
...internal/operators/parallel/ParallelFilterTry.java	100.00% <100.00%> (ø)	6.00 <4.00> (ø)
...3/internal/operators/parallel/ParallelFlatMap.java	100.00% <100.00%> (ø)	5.00 <3.00> (ø)
...al/operators/parallel/ParallelFlatMapIterable.java	100.00% <100.00%> (ø)	5.00 <3.00> (ø)
...internal/operators/parallel/ParallelFromArray.java	100.00% <100.00%> (ø)	5.00 <3.00> (ø)
...rnal/operators/parallel/ParallelFromPublisher.java	100.00% <100.00%> (ø)	4.00 <2.00> (ø)
... and 24 more

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