chore: mark name flag as required

[nitishfy]

This PR does the following changes:

1. The name flag as required for mta helmrelease and mta kustomization command. The --namespace flag has a default value of flux-system namespace, hence if a user never initialize that flag, this flag will never be nil. So there's no point of checking if the --namespace flag has a nil value.
2. Remove the MarkPersistentFlag() from the subcommands.
3. Instead of printing the version in the JSON format, simply print the version in the string format without marshalling. In other words, marshalling is not required when printing the version of mta.

After