Update db:drop_tables to have multi-db versions

What does this MR do and why?

Update rake task drop_tables to support multiple databases. Related to #352770 (closed)

If only a single database is configured, you should continue to see only the singular rake task gitlab:db:drop_tables.

If multiple databases are configured, you should see:

rake -T gitlab:db:drop_tables
rake gitlab:db:drop_tables       # GitLab | DB | Drop all tables
rake gitlab:db:drop_tables:ci    # GitLab | DB | Drop all tables on the ci database
rake gitlab:db:drop_tables:main  # GitLab | DB | Drop all tables on the main database

How to set up and validate locally

If you have multiple databases setup, you can validate against a single database first:

1. Drop all tables on ci database, and verify tables are gone:

   # GITLAB_USE_MODEL_LOAD_BALANCING=true rake gitlab:db:drop_tables:ci
   
   pbair@[local]:5432/gitlabhq_development_ci# \d
             List of relations
    Schema |       Name        | Type  | Owner
   --------+-------------------+-------+-------
    public | schema_migrations | table | pbair
   (1 row)

2. Drop all tables for all databases, and then the tables on main should also be gone:

   # GITLAB_USE_MODEL_LOAD_BALANCING=true rake gitlab:db:drop_tables
   
   pbair@[local]:5432/gitlabhq_development# \d
                List of relations
    Schema |       Name        | Type  | Owner
   --------+-------------------+-------+-------
    public | schema_migrations | table | pbair
   (1 row)

MR acceptance checklist

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