Use Azure HAProxy instances to route Pages data from old Pages IP in Azure to new Pages instance in GCP

At some point, we'll need to get all apex DNS GitLab Pages users to switch their A records over to the new Pages IP address, in GCP.

Because it requires user interaction and synchronisation between the production team (performing the change) and the users, we will need to announce this change at least a month in advance.

However, since this change also carries all the risk of the GCP migration, I would rather avoid setting a date in stone over a month in advance as this will create a deadline and put pressure on the team, which may lead to rushing and mistakes.

For this reason, @northrup proposed leaving the HAProxy instances standing in Azure, routing all pages traffic across the IPSec VPN to the new instance in GCP.

This means that we can announce the switchover to Pages users once the migration is complete, and not put ourselves under unnecessary pressure from an artificial deadline.