feat: introduce lenient maxConcurrency for flatMap

[ozangunalp]

Replenish upstream at inner stream completion even though we hit the concurrency limit

[codecov[bot]]

Codecov Report

:x: Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.00%. Comparing base (a1998df) to head (6746983). :warning: Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	37.50%	4 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2031      +/-   ##
============================================
- Coverage     89.18%   89.00%   -0.19%     
+ Complexity     3120     3114       -6     
============================================
  Files           412      412              
  Lines         13295    13304       +9     
  Branches       1688     1690       +2     
============================================
- Hits          11857    11841      -16     
- Misses          812      825      +13     
- Partials        626      638      +12     

Files with missing lines	Coverage Δ
...n/java/io/smallrye/mutiny/groups/MultiFlatten.java	100.00% <100.00%> (ø)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	88.85% <37.50%> (-2.03%)	:arrow_down:

... and 14 files with indirect coverage changes

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[ozangunalp]

For better coverage some tests need to be duplicated for the lenient concurrency branch

[jponge]

I'm not worried about test coverage here