Now that the official CloudFormation resource `AWS::CertificateManager::Certificate`
(CDK's `Certificate` construct) supports DNS validation we do not want
to recommend using the `DnsValidatedCertificate` construct.

The `DnsValidatedCertificate` construct uses CloudFormation custom
resources to perform the certificate creation and this creates a lot of
maintenance burden on our team (see the list of linked issues).
Currently the primary use case for using `DnsValidatedCertificate` over
`Certificate` is for cross region use cases. For this use case I have
updated the README to have our suggested solution.

fixes #8934, #2914, #20698, #17349, #15217, #14519

…tom-resource