Add drop_enum command for Postgres

[ghiculescu]

Follow up to #41469

This PR adds support for drop_enum, so you can drop enum types explicitly or through a db:migrate:down.

[yahonda]

1. What do you think about replacing DROP TYPE sql statements with drop_enum?

% git grep -n 'DROP TYPE'
activerecord/test/cases/adapters/postgresql/composite_test.rb:35:    @connection.execute "DROP TYPE IF EXISTS full_address"
activerecord/test/cases/adapters/postgresql/enum_test.rb:34:    @connection.execute "DROP TYPE IF EXISTS mood"
activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb:405:        @connection.execute "DROP TYPE IF EXISTS feeling"
activerecord/test/cases/adapters/postgresql/range_test.rb:101:    @connection.execute "DROP TYPE IF EXISTS floatrange"
activerecord/test/cases/adapters/postgresql/range_test.rb:102:    @connection.execute "DROP TYPE IF EXISTS stringrange"
guides/source/active_record_postgresql.md:270:    DROP TYPE article_status;
%

2. How about adding tests with explicit schema name? As we added explicit schema name option for create__enum via Support explicit schemas in PostgreSQL's create_enum #45707.

[ghiculescu]

@yahonda several of those (composite_test and range_test) are not enums, but other examples of custom types. Do you think I should add a more general drop_type? It would do the same thing in Postgres but the name would be more correct.

I made the other changes as suggested - thanks for the schema reminder.

[yahonda]

Thanks for the update.

Do you think I should add a more general drop_type? It would do the same thing in Postgres but the name would be more correct.

No, not necessary. I just git grep but did not verify each lines in detail.

[yahonda]

What do you think about adding if_exists: true support option for drop_enum?

[ghiculescu]

Oh yeah - good idea 👍

Added another review comment.

failing tests

[ghiculescu]

Thanks @yahonda

[yahonda]

Would you address the CHANGELOG.md conflicts?

[ghiculescu]

Yep 👍