Use chpst to force registry user on registry migrations

What does this MR do?

Use chpst to force regstry user on registry migrations

This will facilitate use cases when the user is running as root and calls the registry database commands. This was troublesome in cases where PostgreSQL was running on the same node as the registry and doing peer authentication. Since the peer authentication expects the registry system user instead of root, it would fail the connection or required the user to run the migration command with sudo -u registry or su - registry.

With this change, the users can always call gitlab-ctl registry-database migrate commands without having to prefix it with a user change. This should work for any case.

Changelog: added

Related issues

Closes Documentation for the container registry metada... (gitlab#573653)

The related issue started as a documentation proposal to tell docker users to use su - registry when running the registry migration commands. But after discussing the approach within the issue, Balu proposed that we introduce the chpst approach which makes the user experience simpler, thus avoiding the need to document adding this prefix to the command.

Context: gitlab#573653 (comment 2840012398)

Dependent MR

Fix registry permission to restart service if u... (!8817)

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