Update expiration to preclude negative values.

Co-authored-by: Hännetz <34142036+Haennetz@users.noreply.github.com>

src/api/pki/responses.rs

@@ -6,7 +6,7 @@ use serde::{Deserialize, Serialize};
 pub struct GenerateCertificateResponse {
     pub ca_chain: Option<Vec<String>>,
     pub certificate: String,
-    pub expiration: Option<i64>,
+    pub expiration: Option<u64>,
     pub issuing_ca: String,
     pub private_key: String,
     pub private_key_type: String,