2.x: Add Single.ignoreElement, deprecate toCompletable

[akarnokd]

Rename-deprecate Single.toCompletable to ignoreElement to be in line with the same operator in the other classes and also indicate more clearly in the method name that it ignores the element.

In addition, the marbles have been updated on both Single and Maybe of this operator.

[codecov]

Codecov Report

Merging #5957 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5957      +/-   ##
============================================
+ Coverage     98.23%   98.26%   +0.02%     
- Complexity     6016     6018       +2     
============================================
  Files           656      656              
  Lines         44036    44037       +1     
  Branches       6100     6100              
============================================
+ Hits          43259    43272      +13     
+ Misses          238      226      -12     
  Partials        539      539

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	143 <1> (+1)	⬆️
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	91.5% <0%> (-3.27%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-2.24%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-1.61%)	5% <0%> (ø)
... and 23 more

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[vanniktech]

We deprecate a method that's stable and then add a new one that should be used that's experimental.

Does not seem right to me.

[akarnokd]

I'd be completely fine with ignoreElement being standard right away.

[vanniktech]

Then let's do it.