New yii\helpers\ArrayHelper::flatten() method

[xcopy]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌

As promised in #20297 (comment)

Changes:

• added new flatten() method in yii\helpers\BaseArrayHelper class
• removed yii\log\Target::flatten() (and replaced by ArrayHelper::flatten())

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.84%. Comparing base (3fc7e71) to head (cc0351c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #20306   +/-   ##
=========================================
  Coverage     64.84%   64.84%           
- Complexity    11422    11424    +2     
=========================================
  Files           431      431           
  Lines         37167    37172    +5     
=========================================
+ Hits          24101    24106    +5     
  Misses        13066    13066           

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[samdark]

Thank you!