4.x: Intro Schedulers.virtual(), deprecate Schedulers.io(), fix tests

[akarnokd]

Also expand virtualCreate and virtualTransform to use a Schedulers.virtual() or custom Scheduler.

The virtual thread integration should be largely complete with it.

Resolves #7763

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.575 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

config/checkstyle/checkstyle.xml: 5.122
src/jmh/java/io/reactivex/rxjava4/core/MemoryPerf.java: 4.452
src/main/java/io/reactivex/rxjava4/core/Completable.java: 4.833
src/main/java/io/reactivex/rxjava4/core/Flowable.java: 4.779
src/main/java/io/reactivex/rxjava4/core/Maybe.java: 4.839
src/main/java/io/reactivex/rxjava4/core/Observable.java: 4.758
src/main/java/io/reactivex/rxjava4/core/Scheduler.java: 4.551
src/main/java/io/reactivex/rxjava4/core/Single.java: 4.829
src/main/java/io/reactivex/rxjava4/core/VirtualGenerator.java: 4.919
src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java: 4.821
src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java: 4.821
src/main/java/io/reactivex/rxjava4/core/docs/FlowableDocBasic.java: 4.908
src/main/java/io/reactivex/rxjava4/core/package-info.java: 5.018
src/main/java/io/reactivex/rxjava4/exceptions/OnErrorNotImplementedException.java: 4.874
src/main/java/io/reactivex/rxjava4/exceptions/UndeliverableException.java: 4.959
src/main/java/io/reactivex/rxjava4/internal/functions/Functions.java: 4.748
src/main/java/io/reactivex/rxjava4/internal/observers/DeferredScalarDisposable.java: 4.765
src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableBuffer.java: 4.115
src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCreate.java: 3.943
src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableDistinctUntilChanged.java: 4.095

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

❌ Patch coverage is 62.85714% with 104 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (4.x@a1741d5). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...internal/schedulers/DeferredExecutorScheduler.java	48.46%	89 Missing and 12 partials ⚠️
...va/io/reactivex/rxjava4/schedulers/Schedulers.java	81.81%	2 Missing ⚠️
...va/io/reactivex/rxjava4/plugins/RxJavaPlugins.java	96.77%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (62.85%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@          Coverage Diff           @@
##             4.x    #8051   +/-   ##
======================================
  Coverage       ?   99.30%           
  Complexity     ?     6828           
======================================
  Files          ?      764           
  Lines          ?    48246           
  Branches       ?     6470           
======================================
  Hits           ?    47911           
  Misses         ?      207           
  Partials       ?      128           

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