3.x: Migrate Disposables static factories to Disposable interface

[JakeWharton]

No description provided.

[akarnokd]

I think these additions would count as @since 3.0.0.

[JakeWharton]

Yep. Done.

[codecov]

Codecov Report

Merging #6781 into 3.x will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6781      +/-   ##
============================================
+ Coverage     98.21%   98.26%   +0.04%     
+ Complexity     6257     6256       -1     
============================================
  Files           690      690              
  Lines         45220    45218       -2     
  Branches       6220     6220              
============================================
+ Hits          44414    44434      +20     
+ Misses          283      268      -15     
+ Partials        523      516       -7

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/reactivex/rxjava3/core/Scheduler.java	100% <ø> (ø)	11 <0> (ø)	⬇️
...ava3/internal/operators/maybe/MaybeFromFuture.java	100% <100%> (ø)	8 <7> (ø)	⬇️
...a3/internal/operators/maybe/MaybeFromCallable.java	95% <100%> (ø)	6 <4> (ø)	⬇️
.../rxjava3/internal/operators/single/SingleJust.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...io/reactivex/rxjava3/schedulers/TestScheduler.java	100% <100%> (ø)	11 <0> (ø)	⬇️
...a3/internal/operators/maybe/MaybeFromSupplier.java	85% <100%> (ø)	5 <3> (ø)	⬇️
...eactivex/rxjava3/disposables/SerialDisposable.java	100% <100%> (ø)	8 <0> (ø)	⬇️
...operators/completable/CompletableFromSupplier.java	100% <100%> (ø)	4 <3> (ø)	⬇️
...l/operators/completable/CompletableFromAction.java	100% <100%> (ø)	4 <3> (ø)	⬇️
.../internal/operators/single/SingleFromSupplier.java	100% <100%> (ø)	5 <4> (ø)	⬇️
... and 33 more

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