With the 0.11 release, the deprecated `spec.http01`/`spec.dns01` syntax was removed from the `Issuer`/`ClusterIssuer` CRDs.
Update some references to the old syntax and add a note to the 0.11 upgrade doc about an error one might see if they were still using the old syntax, as was still being provided in the `Issuers` docs until this PR...