Improve error message when old cluster config file exists

[jiayiwang7]

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Description of changes:

Testing (if applicable):

Documentation added/planned (if applicable):

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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 73.63%. Comparing base (4583834) to head (dae98c0).
Report is 257 commits behind head on main.

Files	Patch %	Lines
cmd/eksctl-anywhere/cmd/createcluster.go	0.00%	2 Missing ⚠️
pkg/kubeconfig/kubeconfig.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7748      +/-   ##
==========================================
+ Coverage   73.48%   73.63%   +0.14%     
==========================================
  Files         579      588       +9     
  Lines       36357    37145     +788     
==========================================
+ Hits        26718    27350     +632     
- Misses       7875     8002     +127     
- Partials     1764     1793      +29     

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