4.x: Maybe API reduction: concat, concatEager, merge

[akarnokd]

Simplify API by changing the number of overloads of various operators.

Maybe

• concat and concatDelayError -> concat with MaybeConcatConfig configuration record.
• concatArray and concatArrayDelayError -> concatArray with MaybeConcatConfig configuration record.
• concatEager and concatEagerDelayError -> concatEager with MaybeConcatEagerConfig configuration record.
• concatArrayEager and concatArrayEagerDelayError -> concatArrayEager with MaybeConcatEagerConfig configuration record.
• merge and mergeDelayError -> merge with MaybeMergeConfig configuration record.
• mergeArray and mergeArrayDelayError -> mergeArray with MaybeMergeConfig configuration record.
• Removed the 2, 3 and 4 argument merge|mergeDelayError variants, use mergeArray

Related: #8173

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.791 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Single.java: 4.832
src/main/java/io/reactivex/rxjava4/core/config/SingleMergeConfig.java: 4.859
src/test/java/io/reactivex/rxjava4/core/config/SingleMergeConfigTest.java: 4.990
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableCombineLatestTest.java: 4.693
src/test/java/io/reactivex/rxjava4/internal/operators/single/SingleMergeArrayTest.java: 4.941
src/test/java/io/reactivex/rxjava4/internal/operators/single/SingleMergeTest.java: 4.632
src/test/java/io/reactivex/rxjava4/single/SingleTest.java: 4.548
src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java: 4.835

✅ 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 98.34%. Comparing base (76ecd9a) to head (3074524).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8169      +/-   ##
============================================
+ Coverage     98.33%   98.34%   +0.01%     
- Complexity     6945     6946       +1     
============================================
  Files           778      781       +3     
  Lines         49047    49054       +7     
  Branches       6595     6600       +5     
============================================
+ Hits          48229    48242      +13     
  Misses          625      625              
+ Partials        193      187       -6     

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