3.x: Add Maybe.dematerialize

[akarnokd]

Add the dematerialize operator to Maybe to be in sync with the other classes.

Related #6852

[codecov]

Codecov Report

Merging #6871 into 3.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6871      +/-   ##
============================================
+ Coverage     98.27%   98.29%   +0.01%     
- Complexity     6446     6447       +1     
============================================
  Files           722      723       +1     
  Lines         46783    46817      +34     
  Branches       6401     6404       +3     
============================================
+ Hits          45977    46019      +42     
+ Misses          289      282       -7     
+ Partials        517      516       -1

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	160 <0> (ø)	⬇️
...3/internal/operators/maybe/MaybeDematerialize.java	100% <100%> (ø)	2 <2> (?)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	188 <1> (+1)	⬆️
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0%> (-5.64%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	96.11% <0%> (-1.3%)	6% <0%> (ø)
...ava3/internal/operators/maybe/MaybeMergeArray.java	96.04% <0%> (-1.13%)	6% <0%> (ø)
... and 17 more

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