Disconnect connections used for acquiring advisory lock in migration.

Not disconnecting the connections in the pool results in a session left idling in the DB.

An idle session will prevent the database being dropped and affect
commands like `bin/rails db:test:prepare`. While this is an edge case,
it is still good practice to clean up the connections in the pool.

activerecord/lib/active_record/migration.rb

@@ -1397,6 +1397,8 @@ module ActiveRecord
         )
 
         pool.with_connection { |connection| yield(connection) }
+      ensure
+        pool&.disconnect!
       end
 
       MIGRATOR_SALT = 2053462845

activerecord/test/cases/migration_test.rb

@@ -933,6 +933,24 @@ class MigrationTest < ActiveRecord::TestCase
       end
     end
 
+    if current_adapter?(:PostgreSQLAdapter)
+      def test_with_advisory_lock_closes_connection
+        migration = Class.new(ActiveRecord::Migration::Current) {
+          def version; 100 end
+          def migrate(x)
+          end
+        }.new
+
+        migrator = ActiveRecord::Migrator.new(:up, [migration], @schema_migration, 100)
+
+        query = "SELECT COUNT(*) FROM pg_stat_activity WHERE datname = '#{ActiveRecord::Base.connection_db_config.database}'"
+
+        assert_no_changes -> { ActiveRecord::Base.connection.exec_query(query).rows.flatten.first } do
+          migrator.migrate
+        end
+      end
+    end
+
     def test_with_advisory_lock_raises_the_right_error_when_it_fails_to_release_lock
       migration = Class.new(ActiveRecord::Migration::Current).new
       migrator = ActiveRecord::Migrator.new(:up, [migration], @schema_migration, 100)