4.x: Observable.concat API reduction

[akarnokd]

Reduce API surface by introducing configuration record to various operators.

Observable

• concat + concatDelayError -> concat + ObservableConcatConfig
• concatArray + concatArrayDelayError -> concatArray + ObservableConcatConfig
• removed the 2, 3 and 4 argument concat methods -> use concatArray

Related: #8173

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.817 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Observable.java: 4.759
src/main/java/io/reactivex/rxjava4/core/config/ObservableCombineLatestConfig.java: 4.814
src/test/java/io/reactivex/rxjava4/core/config/ObservableCombineLatestConfigTest.java: 4.996
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableCombineLatestTest.java: 4.680
src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java: 4.838

✅ No secrets or suspicious high-entropy strings found.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (67fd64f) to head (2b4ebb1).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8175      +/-   ##
============================================
- Coverage     98.34%   98.32%   -0.02%     
+ Complexity     6944     6940       -4     
============================================
  Files           782      783       +1     
  Lines         49057    49044      -13     
  Branches       6599     6596       -3     
============================================
- Hits          48246    48224      -22     
- Misses          622      628       +6     
- Partials        189      192       +3     

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