Follow-up from "Migrate Gitaly chart to external Gitaly service" - use project move API for migration

The docs MR !3209 (merged) will introduce a method to migrate from the Gitaly chart to an external Gitaly service using a backup restore approach.

This follow up is to determine how we can use the Project Move API instead, as that would allow us to complete the migration with zero-downtime.

The following discussion from !3209 (merged) should be addressed:

• @Alexand started a discussion:

  • Thank you for the review and tips, @samihiltunen.

    An alternative approach could be to use the Project Move API. So one would first configure the new external storages along the existing ones, move the repositories there, and then remove the old ones. That would likely be easier if the chart supports configuring external Gitaly nodes along the chart configured ones.

    Sounds like a neat approach! Although, I think it's not 100% clear, but it looks like we don't support this. From our Gitaly chart docs we:

    If deploying Gitaly separately, global.gitaly.enabled needs to be set to false and additional configuration will need to be performed as described in the external Gitaly documentation.

    That said, I'm also ok with starting with this approach proposed by @anton, and open a follow-up to test whether we could use the Project Move API, and if necessary open another follow-up to add support for built-in Gitaly to run alongside external ones.

    @anton, I'll let you decide whether you'd like to follow with this proposal or attempt the Project Move API approach. For now I'll stick to reviewing the current approach, but feel free to change direction, just let me know.