praefect: list-untracked-repositories sub-command migrated

The 'list-untracked-repositories' sub-command migrated from the old implementation to the new approach with usage of the third party library. The invocation of the command remains the same, but it has more descriptive representation when help is invoked. Also, it is possible now to invoke help only for the sub-command to see more details about it and its usage.

The 'gitaly_test_sha256' tag removed and tests in subcmd_list_untracked_repositories_test.go run always.

Part of: #5001 (closed)