Remove :timestamp column type

… skip]

…one`

In rails#21126 it was suggested to make "timestamp with time zone" the default type for datetime columns in PostgreSQL. This is in line with PostgreSQL [best practices](https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_timestamp_.28without_time_zone.29). This PR lays some groundwork for that.

This PR adds a configuration option, `ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type`. The default is `:timestamp` which preserves current Rails behavior of using "timestamp without time zone" when you do `t.datetime` in a migration. If you change it to `:timestamptz`, you'll get "timestamp with time zone" columns instead.

If you change this setting in an existing app, you should immediately call `bin/rails db:migrate` to ensure your `schema.rb` file remains correct. If you do so, then existing columns will not be impacted, so for example if you have an app with a mixture of both types of columns, and you change the config, schema dumps will continue to output the correct types.

This PR also adds two new types that can be used in migrations: `t.timestamp` and `t.timestamptz`.

```ruby
ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type = :timestamp # default value is :timestamp

create_table("foo1") do |t|
  t.datetime :default_format # "timestamp without time zone"
  t.timestamp :without_time_zone # "timestamp without time zone"
  t.timestamptz :with_time_zone # "timestamp with time zone"
end

ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type = :timestamptz

create_table("foo2") do |t|
  t.datetime :default_format # "timestamp with time zone" <-- note how this has changed!
  t.timestamp :without_time_zone # "timestamp without time zone"
  t.timestamptz :with_time_zone # "timestamp with time zone"
end

ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::NATIVE_DATABASE_TYPES[:my_custom_type] = { name: "custom_datetime_format_i_invented" }
ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type = :my_custom_type

create_table("foo3") do |t|
  t.datetime :default_format # "custom_datetime_format_i_invented"
  t.timestamp :without_time_zone # "timestamp without time zone"
  t.timestamptz :with_time_zone # "timestamp with time zone"
end
```

**Notes**

- This PR doesn't change the default `datetime` format. The default is still "timestamp without time zone". A future PR could do that, but there was enough code here just getting the config option right.
- See also rails#41395 which set some groundwork (and added some tests) for this.
- This reverts some of rails#15184. rails#15184 alluded to issues in XML serialization, but I couldn't find any related tests that this broke.

…one`

In rails#21126 it was suggested to make "timestamp with time zone" the default type for datetime columns in PostgreSQL. This is in line with PostgreSQL [best practices](https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_timestamp_.28without_time_zone.29). This PR lays some groundwork for that.

This PR adds a configuration option, `ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type`. The default is `:timestamp` which preserves current Rails behavior of using "timestamp without time zone" when you do `t.datetime` in a migration. If you change it to `:timestamptz`, you'll get "timestamp with time zone" columns instead.

If you change this setting in an existing app, you should immediately call `bin/rails db:migrate` to ensure your `schema.rb` file remains correct. If you do so, then existing columns will not be impacted, so for example if you have an app with a mixture of both types of columns, and you change the config, schema dumps will continue to output the correct types.

This PR also adds two new types that can be used in migrations: `t.timestamp` and `t.timestamptz`.

```ruby
ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type = :timestamp # default value is :timestamp

create_table("foo1") do |t|
  t.datetime :default_format # "timestamp without time zone"
  t.timestamp :without_time_zone # "timestamp without time zone"
  t.timestamptz :with_time_zone # "timestamp with time zone"
end

ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type = :timestamptz

create_table("foo2") do |t|
  t.datetime :default_format # "timestamp with time zone" <-- note how this has changed!
  t.timestamp :without_time_zone # "timestamp without time zone"
  t.timestamptz :with_time_zone # "timestamp with time zone"
end

ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::NATIVE_DATABASE_TYPES[:my_custom_type] = { name: "custom_datetime_format_i_invented" }
ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.datetime_type = :my_custom_type

create_table("foo3") do |t|
  t.datetime :default_format # "custom_datetime_format_i_invented"
  t.timestamp :without_time_zone # "timestamp without time zone"
  t.timestamptz :with_time_zone # "timestamp with time zone"
end
```

**Notes**

- This PR doesn't change the default `datetime` format. The default is still "timestamp without time zone". A future PR could do that, but there was enough code here just getting the config option right.
- See also rails#41395 which set some groundwork (and added some tests) for this.
- This reverts some of rails#15184. rails#15184 alluded to issues in XML serialization, but I couldn't find any related tests that this broke.