3.x: CompositeException.printStackTrace to write directly into PrintStream/PrintWriter

[akarnokd]

I don't remember why the stacktrace it wasn't printed directly via the PrintStream/PrintWriter; no reason to aggregate the entire string output.

Fixes: #7211

[codecov]

Codecov Report

Merging #7212 (993e815) into 3.x (5c0ca03) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7212      +/-   ##
============================================
- Coverage     99.54%   99.49%   -0.05%     
+ Complexity     6759     6756       -3     
============================================
  Files           747      747              
  Lines         47392    47391       -1     
  Branches       6382     6382              
============================================
- Hits          47178    47154      -24     
- Misses           96      109      +13     
- Partials        118      128      +10     

Impacted Files	Coverage Δ	Complexity Δ
...activex/rxjava3/exceptions/CompositeException.java	100.00% <100.00%> (ø)	35.00 <6.00> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	10.00% <0.00%> (-1.00%)
...l/operators/observable/ObservableFlatMapMaybe.java	94.36% <0.00%> (-2.82%)	2.00% <0.00%> (ø%)
...a/io/reactivex/rxjava3/subjects/SingleSubject.java	97.53% <0.00%> (-2.47%)	37.00% <0.00%> (-1.00%)
...ternal/operators/observable/ObservablePublish.java	97.29% <0.00%> (-1.81%)	16.00% <0.00%> (-1.00%)
.../operators/observable/ObservableFlatMapSingle.java	95.23% <0.00%> (-1.59%)	2.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.81% <0.00%> (-1.56%)	2.00% <0.00%> (ø%)
...vable/ObservableFlatMapCompletableCompletable.java	98.48% <0.00%> (-1.52%)	3.00% <0.00%> (ø%)
...ators/observable/ObservableFlatMapCompletable.java	98.52% <0.00%> (-1.48%)	2.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	2.00% <0.00%> (ø%)
... and 12 more

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