3.x: widen throws on the XOnSubscribe interfaces

[akarnokd]

This PR widens the throws Exception to throws Throwable on the callback interfaces of the create methods: {Flowable|Observable|Maybe|Single|Completable}OnSubscribe.

[codecov]

Codecov Report

Merging #6579 into 3.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6579      +/-   ##
============================================
+ Coverage     98.21%   98.22%   +0.01%     
+ Complexity     6184     6183       -1     
============================================
  Files           678      678              
  Lines         44900    44900              
  Branches       6211     6211              
============================================
+ Hits          44099    44104       +5     
- Misses          263      265       +2     
+ Partials        538      531       -7

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/reactivex/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.12% <0%> (-1.62%)	6% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.28% <0%> (-0.84%)	50% <0%> (ø)
...reactivex/internal/queue/SpscLinkedArrayQueue.java	96.82% <0%> (-0.8%)	38% <0%> (-1%)
...vex/internal/operators/flowable/FlowableCache.java	97.72% <0%> (-0.76%)	37% <0%> (-1%)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.56%)	3% <0%> (ø)
... and 15 more

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