3.x: Add missing annotations, fix many diamonds

[akarnokd]

• Improve validation code and extend them to other classes/sources.
• Add @NonNull annotations
• Add @SafeVarargs annotations
• Add @CheckReturnValue annotations
• Add more @BackpressureSupport & @SchedulerSupport annotations.
• Add backpressure and scheduler descriptions in ParallelFlowable
• Remove now unnecessary @SuppressWarnings("unchecked")
• Fix wording in some javadocs
• Move toFunction nullcheck into the caller method so the validation refers to the proper parameter name
• Use diamond arguments where possible
• Remove unused type arguments.

[codecov]

Codecov Report

Merging #6795 into 3.x will increase coverage by 0.01%.
The diff coverage is 99.47%.

@@             Coverage Diff              @@
##                3.x    #6795      +/-   ##
============================================
+ Coverage     98.26%   98.27%   +0.01%     
+ Complexity     6284     6283       -1     
============================================
  Files           696      696              
  Lines         45395    45427      +32     
  Branches       6233     6233              
============================================
+ Hits          44607    44644      +37     
+ Misses          276      272       -4     
+ Partials        512      511       -1

Impacted Files	Coverage Δ	Complexity Δ
...eactivex/rxjava3/internal/functions/Functions.java	99.12% <ø> (-0.03%)	37 <0> (ø)
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	537 <0> (ø)	⬇️
...tivex/rxjava3/disposables/CompositeDisposable.java	100% <100%> (+1.85%)	40 <0> (+1)	⬆️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	177 <3> (ø)	⬇️
...ctivex/rxjava3/processors/SerializedProcessor.java	97.87% <100%> (ø)	28 <0> (ø)	⬇️
...io/reactivex/rxjava3/schedulers/TestScheduler.java	100% <100%> (ø)	11 <1> (ø)	⬇️
...vex/rxjava3/observables/ConnectableObservable.java	100% <100%> (ø)	12 <1> (ø)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <100%> (-2.44%)	51 <5> (ø)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	97.51% <100%> (ø)	50 <10> (ø)	⬇️
... and 46 more

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