Use Backup::DatabaseConnection if configured

What does this MR do and why?

Use Backup::DatabaseConnection if configured

If user runs the db rake task with ENV config for direct connection to Postgres, then use it with Backup::DatabaseConnection.

References

• Provide a way to bypass PgBouncer for db mainte... (#562640 - closed)

How to set up and validate locally

1. Checkout this MR

2. Run the task: bin/rails gitlab:db:collation_checker:main, it should run without error

3. Run the task again with correct ENV variable set:

   $ GITLAB_BACKUP_PGUSER=bishwa bin/rails gitlab:db:collation_checker:main
   I, [2025-08-21T18:09:26.787652 #90125]  INFO -- : Checking for PostgreSQL collation mismatches on main database...
   I, [2025-08-21T18:09:26.792673 #90125]  INFO -- : No collation version mismatches detected on main.
   I, [2025-08-21T18:09:26.792692 #90125]  INFO -- : Found 8 indexes to corruption spot check.
   I, [2025-08-21T18:09:26.839257 #90125]  INFO -- : No corrupted indexes detected.

4. To test that it respects the ENV variable, set the wrong value and assert that it fails

   $ GITLAB_BACKUP_PGUSER=foobar bin/rails gitlab:db:collation_checker:main
   I, [2025-08-21T18:10:19.240188 #90201]  INFO -- : Checking for PostgreSQL collation mismatches on main database...
   bin/rails aborted!
   ActiveRecord::DatabaseConnectionError: There is an issue connecting to your database with your username/password, username: foobar.

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Related to #562640 (closed)