Update error msgs for OSImageURL validation:

[jacobweinstock]

Issue #, if available:

Description of changes: As the error message is provided to Users, including the expected Kubernetes version format to the error message allows to User to clearly understand how to resolve the validation error.

Testing (if applicable):

Documentation added/planned (if applicable):

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.64%. Comparing base (4583834) to head (f6a54f5). Report is 257 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7769      +/-   ##
==========================================
+ Coverage   73.48%   73.64%   +0.15%     
==========================================
  Files         579      588       +9     
  Lines       36357    37207     +850     
==========================================
+ Hits        26718    27400     +682     
- Misses       7875     8015     +140     
- Partials     1764     1792      +28     

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[jacobweinstock]

/approve

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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