Geo: Refactor backup task moving methods to Backup module

What does this MR do and why?

This MR refactors the backup.rake task moving methods from backup namespace to Tasks::Gitlab::Backup module context defined in !119004 (merged). It also extracts duplicated logic from the task to private class methods.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #410070 (closed)