Logs: describe_log_groups() now returns the logStreamArn-property

[bblommers]

Fixes #8371

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.62%. Comparing base (9fa5890) to head (4de16c4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
moto/logs/models.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8442   +/-   ##
=======================================
  Coverage   92.62%   92.62%           
=======================================
  Files        1225     1225           
  Lines      105633   105646   +13     
=======================================
+ Hits        97843    97856   +13     
  Misses       7790     7790           

Flag	Coverage Δ
servertests	27.75% <10.52%> (-0.01%)	⬇️
unittests	92.59% <94.73%> (+<0.01%)	⬆️

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