3.x: Add concatMapX operators (aliases)

[akarnokd]

Add the following operator aliases:

• Maybe.concatMapCompletable
• Maybe.concatMapSingle
• Single.concatMapCompletable
• Single.concatMap
• Single.concatMapMaybe

Related #6852

In addition, some missing or wrong Maybe marbles have been fixed:

[codecov]

Codecov Report

Merging #6879 into 3.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6879      +/-   ##
============================================
+ Coverage     98.24%   98.25%   +0.01%     
- Complexity     6534     6536       +2     
============================================
  Files           735      735              
  Lines         47148    47153       +5     
  Branches       6432     6432              
============================================
+ Hits          46319    46330      +11     
+ Misses          303      302       -1     
+ Partials        526      521       -5

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	175 <3> (+3)	⬆️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	202 <3> (+2)	⬆️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.7% <0%> (-3.65%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	94.17% <0%> (-3.24%)	6% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0%> (-1.39%)	56% <0%> (-1%)
...3/internal/operators/flowable/FlowablePublish.java	97.53% <0%> (-0.99%)	16% <0%> (-1%)
...3/internal/operators/flowable/FlowableGroupBy.java	95.59% <0%> (-0.95%)	3% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0%> (-0.91%)	2% <0%> (ø)
... and 18 more

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