3.x: Add Completable.onErrorReturn[Item]

[akarnokd]

The operators were already available elsewhere.

Related #6852

In addition, the Maybe variants have received updated marbles as well:

Related #5806

[codecov]

Codecov Report

Merging #6886 into 3.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6886      +/-   ##
============================================
- Coverage     98.26%   98.23%   -0.03%     
- Complexity     6569     6573       +4     
============================================
  Files           735      736       +1     
  Lines         47204    47235      +31     
  Branches       6432     6433       +1     
============================================
+ Hits          46384    46402      +18     
- Misses          298      310      +12     
- Partials        522      523       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	209 <0> (ø)	⬇️
...3/internal/operators/maybe/MaybeOnErrorReturn.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <100%> (ø)	139 <2> (+2)	⬆️
...perators/completable/CompletableOnErrorReturn.java	100% <100%> (ø)	2 <2> (?)
...a3/internal/operators/flowable/FlowableCreate.java	90.29% <0%> (-6.15%)	6% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-2.91%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	92.95% <0%> (-2.12%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.11%)	3% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98.18% <0%> (-1.82%)	3% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
... and 20 more

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