3.x: constrain upstream requests from take, remove limit operator

[davidmoten]

As per discussion in #6569, this PR constrains upstream requests from the take operator and removes the limit operator.

I've also added a couple of unit tests for coverage of elementAt that I didn't include in #6620.

[codecov]

Codecov Report

Merging #6650 into 3.x will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6650      +/-   ##
============================================
- Coverage     98.11%   98.08%   -0.04%     
+ Complexity     6191     6186       -5     
============================================
  Files           678      677       -1     
  Lines         44722    44674      -48     
  Branches       6176     6166      -10     
============================================
- Hits          43880    43818      -62     
- Misses          294      302       +8     
- Partials        548      554       +6

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	559 <0> (-2)	⬇️
...ava3/internal/operators/flowable/FlowableTake.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-4.93%)	2% <0%> (ø)
...al/operators/flowable/FlowableMergeWithSingle.java	97.59% <0%> (-2.41%)	2% <0%> (ø)
...nal/operators/flowable/FlowableMergeWithMaybe.java	97.66% <0%> (-2.34%)	2% <0%> (ø)
...va3/internal/operators/flowable/FlowableRange.java	95.87% <0%> (-2.07%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	97.29% <0%> (-1.81%)	16% <0%> (-1%)
...nal/operators/flowable/FlowableConcatMapEager.java	97.07% <0%> (-1.76%)	2% <0%> (ø)
...ava3/internal/operators/maybe/MaybeMergeArray.java	96.04% <0%> (-1.7%)	6% <0%> (ø)
... and 30 more

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[akarnokd]

Why not use the limit() internals?

[akarnokd]

The Javadoc of take should contain these extra information.

[akarnokd]

Why not simply delete take and rename limit? That way, the tests and documentation of limit would remain intact.

[davidmoten]

Indeed! Will do.

[davidmoten]

Copied code from limit to take and fixed javadoc.