Add a deprecation warning to follow certbot deprecation.

[JulienPalard]

Closes #23

This is what it gives with the old prefixed syntax (note that it still works, but there's the two warnings):

$ certbot certonly --cert-name example.com -n --agree-tos -d example.com -m admin@example.com --authenticator certbot-plugin-gandi:dns --certbot-plugin-gandi:dns-credentials /etc/gandi.ini --rsa-key-size 4096 --logs-dir ./ --work-dir ./ --config-dir ./
Saving debug log to /home/mdk/clones/certbot/letsencrypt.log
Plugin legacy name certbot-plugin-gandi:dns may be removed in a future version. Please use dns instead.
Certbot is moving to remove 3rd party plugins prefixes.

Please use --authenticator dns-gandi --dns-gandi-credentials

See: https://github.com/certbot/certbot/pull/8131

Plugins selected: Authenticator certbot-plugin-gandi:dns, Installer None

This is what it gives with the new unprefixed syntax, but the old plugin name (note that it still works, single warning):

$ certbot certonly --cert-name example.com -n --agree-tos -d example.com -m admin@example.com --authenticator dns --dns-credentials /etc/gandi.ini --rsa-key-size 4096 --logs-dir ./ --work-dir ./ --config-dir ./
Saving debug log to /home/mdk/clones/certbot/letsencrypt.log
Certbot is moving to remove 3rd party plugins prefixes.

Please use --authenticator dns-gandi --dns-gandi-credentials

See: https://github.com/certbot/certbot/pull/8131

Plugins selected: Authenticator dns, Installer None

This is what it gives with the new syntax (no warnings):

$ certbot certonly --cert-name example.com -n --agree-tos -d example.com -m admin@example.com --authenticator dns-gandi --dns-gandi-credentials /etc/gandi.ini --rsa-key-size 4096 --logs-dir ./ --work-dir ./ --config-dir ./
Saving debug log to /home/mdk/clones/certbot/letsencrypt.log

Plugins selected: Authenticator dns-gandi, Installer None

[obynio]

This looks fine to me, I'll merge it, update the doc and draft a new release.