2.x: Apply promotions for v2.1

[akarnokd]

This PR applies the promotions listed in #5243.

Suggested review strategy:

• Check if the @Experimental annotation has been removed or turned into @Beta.
• Check if the <p>History: 2.0.x matches the original value of the updated @since 2.0.x.

[codecov]

Codecov Report

Merging #5307 into 2.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5307      +/-   ##
============================================
- Coverage     96.09%   96.02%   -0.07%     
+ Complexity     5774     5768       -6     
============================================
  Files           630      630              
  Lines         41148    41146       -2     
  Branches       5714     5714              
============================================
- Hits          39542    39512      -30     
- Misses          630      668      +38     
+ Partials        976      966      -10

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/reactivex/subjects/SingleSubject.java	95.23% <ø> (-2.39%)	38 <0> (-1)
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
.../java/io/reactivex/subscribers/TestSubscriber.java	99.27% <ø> (ø)	57 <0> (ø)	⬇️
...n/java/io/reactivex/parallel/ParallelFlowable.java	100% <ø> (ø)	49 <0> (ø)	⬇️
.../main/java/io/reactivex/plugins/RxJavaPlugins.java	100% <ø> (ø)	146 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	112 <0> (ø)	⬇️
...activex/exceptions/ProtocolViolationException.java	100% <ø> (ø)	1 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	168 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	99.33% <ø> (ø)	132 <0> (ø)	⬇️
...java/io/reactivex/subjects/CompletableSubject.java	94.8% <ø> (ø)	35 <0> (ø)	⬇️
... and 61 more

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