3.x: Promote Beta/Experimental to Standard

[akarnokd]

Promote the following experimental operators to standard:

• Completable.delaySubscription(long, TimeUnit [, Scheduler])
• Completable.materialize()
• Flowable.dematerialize(Function)
• Maybe.doOnTerminate()
• Maybe.materialize()
• Observable.dematerialize(Function)
• Single.dematerialize(Function)
• Single.materialize()

There were no Beta operators in 2.2.x.

[codecov]

Codecov Report

Merging #6537 into 3.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## 3.x #6537 +/- ## =========================================== + Coverage 98.2% 98.28% +0.08%  Complexity 6199 6199 =========================================== Files 680 680 Lines 44976 44976 Branches 6202 6202 =========================================== + Hits 44168 44204 +36  + Misses 259 241 -18  + Partials 549 531 -18

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/mixed/MaterializeSingleObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	572 <0> (ø)	⬇️
.../operators/completable/CompletableMaterialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...vex/internal/operators/maybe/MaybeMaterialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	11 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	118 <0> (ø)	⬇️
...x/internal/operators/single/SingleMaterialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	547 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	148 <0> (ø)	⬇️
...internal/operators/single/SingleDematerialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
... and 30 more

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