Remove vapplication.kb.io validation from neco-admission

[zoetrope]

What

In order to prevent tenants from accessing arbitrary repositories, we developed neco-admission to validate app.spec.project according to app.spec.source.repoURL.
https://github.com/cybozu/neco-containers/blob/main/admission/docs/configuration.md#argocdapplicationvalidator

We would like to remove this neco-admission validation and instead use ArgoCD's standard features to resolve the problem.

How

Conduct a PoC and consider how to implement.
ref. argoproj/argo-cd#5417

Checklist

• Finish implementation of the issue
• Test all functions
• Have enough logs to trace activities
• Notify developers of necessary actions