2.x: Fix mergeWith not canceling other when the main fails

[akarnokd]

Fix the same bug in 2.x; mergeWith cancelling the main source when the main source errors instead of the other source.

Fixes #6597
Related #6598

[codecov]

Codecov Report

Merging #6599 into 2.x will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #6599      +/-   ##
===========================================
- Coverage     98.3%   98.26%   -0.05%     
- Complexity    6336     6337       +1     
===========================================
  Files          677      677              
  Lines        45488    45492       +4     
  Branches      6321     6321              
===========================================
- Hits         44719    44703      -16     
- Misses         232      245      +13     
- Partials       537      544       +7

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/flowable/FlowableMergeWithMaybe.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...erators/flowable/FlowableMergeWithCompletable.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...al/operators/flowable/FlowableMergeWithSingle.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...operators/observable/ObservableMergeWithMaybe.java	100% <100%> (+0.89%)	2 <0> (ø)	⬇️
...perators/observable/ObservableMergeWithSingle.java	100% <100%> (+0.93%)	2 <0> (ø)	⬇️
...ors/observable/ObservableMergeWithCompletable.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.33% <0%> (-5.32%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	91.5% <0%> (-4.58%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
... and 30 more

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