3.x: [Java 8] Upgrade to Java 8, add Flowable.fromX operators

[akarnokd]

This PR upgrades RxJava to use and compile with Java 8.

• Build target set to 8
• AnimalSniffer set to 8
• JavaDocs link to the official JDK set to 8
• Implemented operators:
  • Flowable.fromOptional
  • Flowable.fromCompletionStage
  • Flowable.fromStream
• Applied @SafeVarargs to many methods
• Created marbles

Related #6695

[codecov]

Codecov Report

Merging #6765 into 3.x will increase coverage by 0.01%.
The diff coverage is 99.38%.

@@             Coverage Diff              @@
##                3.x    #6765      +/-   ##
============================================
+ Coverage     98.19%   98.21%   +0.01%     
- Complexity     6196     6205       +9     
============================================
  Files           679      681       +2     
  Lines         44622    44784     +162     
  Branches       6138     6164      +26     
============================================
+ Hits          43817    43983     +166     
+ Misses          283      281       -2     
+ Partials        522      520       -2

Impacted Files	Coverage Δ	Complexity Δ
.../reactivex/rxjava3/observers/BaseTestConsumer.java	97.23% <ø> (ø)	69 <0> (ø)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <100%> (ø)	562 <3> (+3)	⬆️
...va3/internal/jdk8/FlowableFromCompletionStage.java	100% <100%> (ø)	2 <2> (?)
...ivex/rxjava3/internal/jdk8/FlowableFromStream.java	99.23% <99.23%> (ø)	5 <5> (?)
...l/operators/observable/ObservableFlatMapMaybe.java	85.91% <0%> (-9.16%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	94.16% <0%> (-2.5%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.18% <0%> (-2.33%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
... and 17 more

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[He-Pin]

With this move, there are many we can help.

[He-Pin]

Should remove the usage of Optional.of with Optional.ofNullable.