Add default case to handle invalid --output flag in version command

[kfess]

Description

Add default case for invalid --output flag in version command

When users specify an invalid value for the --output flag (e.g., --output=yml),
the command fails silently with no error message or output.

This PR adds a default case to the switch statement that returns a clear error message,
matching the error handling pattern used in kubectl version.

Output after adding default block

$ kustomize version --output=yml
Error: --output must be 'yaml' or 'json'
Usage:
  kustomize version [flags]

Examples:
kustomize version

Flags:
  -h, --help            help for version
  -o, --output string   One of 'yaml' or 'json'.

Global Flags:
      --stack-trace   print a stack-trace on error

exit status 1

Fixes #6032

Additional context

This is my first contribution to the kustomize codebase.
Please let me know if there's anything I should adjust to better align with the project's conventions.

[k8s-ci-robot]

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This pull-request has been approved by: kfess
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