3.x: Upgrade Gradle & dependency versions

[akarnokd]

This PR upgrades to:

• Gradle 5.6
• TestNG 7.0.0
• Mockito 3.0.0
• JMH 1.21
• JMH plugin 0.5.0-rc-2 (required by Gradle 5.5+)
• License 0.15.0
• Jacoco 0.8.4
• Guava 28

In addition, the osgi plugin has been deprecated in Gradle 5 and has been removed in Gradle 6, therefore a separate plugin called bnd is required to produce the necessary jar properties.
(Also the MockitoJunitRunner has been moved to a new location, hence the package change.)

This Gradle version should support working on a Java 11 runtime when executing gradlew build.

Note that this may require further "live" testing as the auto-release of snapshots as well as auto-release can't be properly tested offline.

[codecov]

Codecov Report

Merging #6632 into 3.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6632      +/-   ##
============================================
- Coverage     98.16%   98.15%   -0.01%     
- Complexity     6187     6189       +2     
============================================
  Files           678      678              
  Lines         44804    44801       -3     
  Branches       6177     6177              
============================================
- Hits          43982    43976       -6     
- Misses          282      289       +7     
+ Partials        540      536       -4

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.14% <0%> (-4.13%)	2% <0%> (ø)
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	97.29% <0%> (-1.81%)	16% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.25% <0%> (-1.15%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.07%)	5% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	94.89% <0%> (-0.73%)	2% <0%> (ø)
...java3/internal/operators/flowable/FlowableZip.java	97.98% <0%> (-0.51%)	6% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.32% <0%> (-0.47%)	3% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	94.31% <0%> (-0.38%)	3% <0%> (ø)
... and 15 more

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[akarnokd]

Snapshotting worked:

• https://travis-ci.org/ReactiveX/RxJava/builds/575569079#L4644
• https://oss.jfrog.org/libs-snapshot/io/reactivex/rxjava3/rxjava/3.0.0-SNAPSHOT/