3.x: Fix experimental, signatures of throttleLast

[akarnokd]

• Add experimental annotations
• Fix Observable.sample wrong overload
• Fix contravariance on the onDropped consumer.

[codecov]

Codecov Report

Merging #7490 (ae560d8) into 3.x (1104c09) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7490      +/-   ##
============================================
- Coverage     99.53%   99.51%   -0.02%     
  Complexity     6792     6792              
============================================
  Files           752      752              
  Lines         47646    47646              
  Branches       6398     6398              
============================================
- Hits          47423    47416       -7     
- Misses          101      108       +7     
  Partials        122      122              

Impacted Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <100.00%> (ø)
...ternal/operators/flowable/FlowableSampleTimed.java	100.00% <100.00%> (ø)
...al/operators/observable/ObservableSampleTimed.java	100.00% <100.00%> (ø)
...x/rxjava3/internal/operators/single/SingleAmb.java	96.61% <0.00%> (-3.39%)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.30% <0.00%> (-2.60%)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	⬇️
...o/reactivex/rxjava3/processors/AsyncProcessor.java	97.91% <0.00%> (-2.09%)	⬇️
...va3/internal/operators/parallel/ParallelRunOn.java	98.53% <0.00%> (-1.47%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	95.07% <0.00%> (-1.41%)	⬇️
... and 10 more

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[vanniktech]

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