Check for policies' `protocol_version` and error if protocol version is not supported.

[viccuad]

Acceptance criteria

Policy server should consume policy_evaluator::evaluator::PolicyEvaluator::protocol_version(), and error if the policy protocol version is not supported.

See kwctl for an example: https://github.com/kubewarden/kwctl/blob/23c4eae60d5a687bc64382b2cae3cd725e529637/src/backend.rs#L52-L61