Refactor decomposition migration to use Backup::DatabaseConfiguration

What does this MR do and why?

During the refactor of !137454 (merged) a parallel work happened to create a script for the database decomposition (!135585 (merged)) that relied on the Backup::DatabaseModel which was refactored into Backup::DatabaseConfiguration and Backup::DatabaseConnection.

For some reason the regular MR flow did not catch this and both were eventually merged to master.

The Backup::DatabaseModel was then restored in : !139939 (merged) to ensure master was again functional.

This MR refactors the decomposition migration to use the newly refactored classes and remove again the Backup::DatabaseModel.

Related to: #435529 (closed)

How to set up and validate locally

Using GDK:

1. Modify database.yml: comment out ci section (so Rails is using a single database)
2. Drop and create ci database gdk psql -c "DROP DATABASE gitlabhq_development_ci";gdk psql -c "CREATE DATABASE gitlabhq_development_ci"
3. Run the rake task: bundle exec rake gitlab:db:decomposition:migrate
4. Verify CI database now also have projects: gdk psql -d gitlabhq_development -c "SELECT COUNT(*) FROM projects";gdk psql -d gitlabhq_development_ci -c "SELECT COUNT(*) FROM projects"; should match

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.