cloud-init upgrade causes vultr init networking to fail.

On upgrading an Ubuntu Mantic server on Vultr I started getting an error on boot:

2024-03-23 15:02:27,710 - url_helper.py[WARNING]: Calling 'None' failed [119/120s]: request error [HTTPConnectionPool(host='fd00:ec2::254', port=80): Max retries exceeded with url: /2
009-04-04/meta-data/instance-id (Caused by NewConnectionError(': Failed to establish a new connection: [Errno 101] Network
is unreachable'))]

And now this message appears on login:

**************************************************************************
This system is using the EC2 Metadata Service, but does not appear to
be running on Amazon EC2 or one of cloud-init's known platforms that
provide a EC2 Metadata service. In the future, cloud-init may stop
reading metadata from the EC2 Metadata Service unless the platform can
be identified.

If you are seeing this message, please file a bug against
cloud-init at
https://github.com/canonical/cloud-init/issues
Make sure to include the cloud provider your instance is
running on.

For more information see
https://github.com/canonical/cloud-init/issues/2795

After you have filed a bug, you can disable this warning by
launching your instance with the cloud-config below, or
putting that content into
/etc/cloud/cloud.cfg.d/99-ec2-datasource.cfg

cloud-config
datasource:
Ec2:
strict_id: false
**************************************************************************

Disable the warnings above by:
touch /root/.cloud-warnings.skip
or
touch /var/lib/cloud/instance/warnings/.skip

Any more information you might require please let me know.

Start by reproducing the cloud-init upgrade on an Ubuntu Mantic Vultr server and inspect the boot error and EC2 metadata warning. Trace how cloud-init identifies the platform and reaches the metadata endpoint; done means networking initialization succeeds without the unreachable metadata request or unsupported-platform warning.

Written by the indexing model from the issue text.