Remove obsolete Issuer and Principal classes?

[richardfan1126]

Question

• Given the introduction of new CiProvider in CiProvider as a new OIDCIssuer type #1729
  And the fact that all CI providers have been changed to type: ci-provider in https://github.com/sigstore/fulcio/blob/main/config/identity/config.yaml

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Should we remove the obsolete Issuer and Principal classes under pkg/identity to avoid confusion?

[haydentherapper]

Yes, we should remove these, but I would prefer to hold off until we cut a major release of Fulcio to fully remove these as I don't want to break any existing deployments.

In the meantime, we could mark these providers as deprecated.