chore: use new fake k8s client constructor

[crenshaw-dev]

fake.NewSimpleClientset is deprecated in favor of fake.NewClientset.

[bunnyshell]

❌ Preview Environment deleted from Bunnyshell

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[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@030a7be). Learn more about missing BASE report.
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argocd/commands/admin/cluster.go	0.00%	1 Missing ⚠️
cmd/argocd/commands/admin/repo.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #21186   +/-   ##
=========================================
  Coverage          ?   55.23%           
=========================================
  Files             ?      324           
  Lines             ?    55649           
  Branches          ?        0           
=========================================
  Hits              ?    30738           
  Misses            ?    22276           
  Partials          ?     2635           

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