UB: fix use of `warn` method instead of `warning` for outputing messages

What does this MR do and why?

Fix a method call in the Unified Backup codebase that was renamed to something else.

This change prevents the following stacktrace:

/opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/targets/database.rb:97:in `block in restore': private method `warn' called for Gitlab::Backup::Cli::Output:Module (NoMethodError)

                Gitlab::Backup::Cli::Output.warn(
                                           ^^^^^
Did you mean?  warning
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/targets/database.rb:84:in `each'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/targets/database.rb:84:in `restore'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/tasks/task.rb:36:in `restore!'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/restore_executor.rb:54:in `block (2 levels) in execute_all_tasks'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/restore_executor.rb:82:in `measure_duration'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/restore_executor.rb:53:in `block in execute_all_tasks'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/tasks.rb:42:in `block in build_each'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/tasks.rb:41:in `each'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/tasks.rb:41:in `build_each'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/restore_executor.rb:50:in `execute_all_tasks'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/restore_executor.rb:29:in `execute'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/commands/restore_subcommand.rb:27:in `block in all'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/commands/restore_subcommand.rb:50:in `measure_duration'
        from /opt/gitlab/embedded/service/gitlab-rails/gems/gitlab-backup-cli/lib/gitlab/backup/cli/commands/restore_subcommand.rb:24:in `all'
...
        from /opt/gitlab/embedded/service/gitlab-rails/bin/gitlab-backup-cli:14:in `<main>'

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Related to #461878