Use DROP SCHEMA ... CASCADE in db:drop_tables

When dropping the EXTRA_SCHEMAS in the rake task gitlab:db:drop_tables,
specify a CASCADE as well, in case we have detached partitions which
won't be automatically removed by dropping the parent table.

lib/tasks/gitlab/db.rake

@@ -49,7 +49,7 @@ namespace :gitlab do
 
       # Drop all extra schema objects GitLab owns
       Gitlab::Database::EXTRA_SCHEMAS.each do |schema|
-        connection.execute("DROP SCHEMA IF EXISTS #{connection.quote_table_name(schema)}")
+        connection.execute("DROP SCHEMA IF EXISTS #{connection.quote_table_name(schema)} CASCADE")
       end
     end
 

spec/tasks/gitlab/db_rake_spec.rb

@@ -191,7 +191,7 @@ RSpec.describe 'gitlab:db namespace rake task', :silence_stdout do
 
     it 'drops extra schemas' do
       Gitlab::Database::EXTRA_SCHEMAS.each do |schema|
-        expect(connection).to receive(:execute).with("DROP SCHEMA IF EXISTS \"#{schema}\"")
+        expect(connection).to receive(:execute).with("DROP SCHEMA IF EXISTS \"#{schema}\" CASCADE")
       end
 
       subject