Replace gitaly.client_path with backup.gitaly_backup_path

What does this Merge Request do and why?

As per gitlab!64834 (merged) client_path has actually been removed, so there needs to be a new config to locate the gitaly-backup binary.

It should bring these gitaly related settings in line with gitlab.yml.example

Testing

Reconfigure gdk:

gdk reconfigure

Turn on the feature in gitlab console:

Feature.enable(:gitaly_backup)

See if the backups run:

SKIP=db bundle exec rake gitlab:backup:create

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.