[Phase 3] Drop the obsolete database schema

[dmikhaylovnv]

Description

Remove storage after every running service has stopped accessing it.

Definition of Done

• Remove cluster_groups_by_authorized_accounts from:
• migrations/cassandra/keyspaces/sis_api/03_init_tables.up.sql
• ICMS local_env/cassandra/schema/schema.cql
• Add the idempotent drop migration currently named 11_drop_bart_cluster_groups.up.sql, using the next available migration number when this phase lands.
• Document the prerequisite that phase 2 is fully deployed and the rollback boundary.

This ordering matters because Cassandra migrations can run before the ICMS rollout. Dropping the table in the same release as its code removal can break older pods still using the discovery fallback.