3.x: Upgrade to Gradle 6.0.1

[akarnokd]

Upgrade to Gradle 6.0.1 (supports building with JDK 13).

./gradlew wrapper --gradle-version=6.0.1

(Looks like there was no change to gradle-wrapper.jar.)

[codecov]

Codecov Report

Merging #6741 into 3.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6741      +/-   ##
============================================
- Coverage     98.13%   98.09%   -0.05%     
  Complexity     6188     6188              
============================================
  Files           677      677              
  Lines         44663    44663              
  Branches       6171     6171              
============================================
- Hits          43830    43810      -20     
- Misses          294      305      +11     
- Partials        539      548       +9

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableFlatMapSingle.java	88.88% <0%> (-4.77%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	94.16% <0%> (-2.5%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-1.58%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.66% <0%> (-1.57%)	2% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...rnal/operators/observable/ObservableObserveOn.java	98.69% <0%> (-1.31%)	3% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.07%)	5% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	95.28% <0%> (-1.05%)	3% <0%> (ø)
... and 14 more

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