Truncate migrated sec tables on gitlab_main

What

Upon completion of the rollout and verification that we won't need to rollback to main, we can safely truncate the old sec tables within patroni-main. This will need to happen after we break reverse logical replication and have locked the sec tables within patroni-main.

Tasks

1. Update gitlab:db:truncate_legacy_tables rake task to support sec DB
2. Execute gitlab:db:truncate_legacy_tables:main against GSTG
3. Execute (newly added) gitlab:db:truncate_legacy_tables:sec against GSTG
4. Execute gitlab:db:truncate_legacy_tables:main against GPRD
5. Execute (newly added) gitlab:db:truncate_legacy_tables:sec against GPRD