The certificate can be issued by multiple issuers. letsencrypt-staging, for example, is for testing purposes. In our case, we are more adventurous, so we will directly use the letsencrypt-prod issuer. As usual, use kubectl apply -f certificate-issuer.yaml, which has the following contents:

apiVersion: certmanager.k8s.io/v1alpha1
kind: ClusterIssuer
metadata:
  name: letsencrypt-prod
spec:
  acme:
    server: https://acme-v02.api.letsencrypt.org/directory
    email: <insert your email here>
    privateKeySecretRef:
      name: letsencrypt-prod
    http01: {}