[release-1.14] Set cluster name label for pre-existing kubeconfig

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #4833

/assign willie-yao

Set cluster name label for pre-existing kubeconfig

[nojnhuh]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 8d46d1ecc62b866033454078c91215709dba330d

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 62.65%. Comparing base (7c032dd) to head (3c1bb0a).
Report is 3 commits behind head on release-1.14.

Files	Patch %	Lines
controllers/azuremanagedcontrolplane_reconciler.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.14    #4846      +/-   ##
================================================
- Coverage         62.65%   62.65%   -0.01%     
================================================
  Files               192      192              
  Lines             15690    15694       +4     
================================================
+ Hits               9831     9833       +2     
- Misses             5172     5174       +2     
  Partials            687      687              

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