3.x: disambiguate startWith+1 to startWithItem & startWithIterable

[akarnokd]

startWith(T) and startWith(Iterable) was causing some trouble with startWith(Publisher). This PR will rename the former two.

The diagrams will be updated in a separate PR.

Resolves: #6122

[vanniktech]

This is one of the changes where library authors can run into troubles. The only way to fix this is to use 3.x publish a new version and drop 2.x support

[codecov]

Codecov Report

Merging #6530 into 3.x will decrease coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6530      +/-   ##
============================================
- Coverage     98.23%   98.17%   -0.07%     
+ Complexity     6188     6183       -5     
============================================
  Files           680      680              
  Lines         44883    44883              
  Branches       6193     6193              
============================================
- Hits          44092    44064      -28     
- Misses          248      264      +16     
- Partials        543      555      +12

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	541 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	566 <1> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.78% <0%> (-3.87%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	90.84% <0%> (-2.62%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	98.19% <0%> (-1.81%)	17% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-1.39%)	56% <0%> (-1%)
... and 20 more

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[akarnokd]

These are conveniences for concatArray(wraper(first), this) so those libraries affected can still support both versions with the same method call.