Allow configuring TXT registry to only generate the "new" format of TXT records

[mjlshen]

What would you like to be added: Currently when using the TXT registry, two TXT records (old + new format) are being created for every record the external-dns creates: https://github.com/kubernetes-sigs/external-dns/blob/c45d0de9090669ee9a40487b6fab7b9b98be7baf/registry/txt.go#L212-L242

I would like to propose a flag (or some other way) to configure the TXT registry to only use the new format.

Why is this needed: I am trying to use external-dns to manage DNS records at scale and am running into cloud provider limits, e.g. by default Azure public DNS Zones allow for a maximum of 10,000 record sets. This quota can be increased, but if external-dns can be configured so that the TXT registry only creates one additional record, this would be a massive improvement for my use-case.