Primary keys added in non-initial migration are not created as identity columns in PostgreSQL

[jackcbrown89]

In this example, a model was created initially with a primary key on a one-to-one field. The primary key was then moved to a new AutoField. This was working prior to the 4.1 release.

## models.py
class Example(models.Model):
    old_id = models.OneToOneField(
        OtherModel,
        on_delete=models.CASCADE,
        ## primary_key=True,  <--- used to be the primary key
    )

    id = models.AutoField(primary_key=True) # <--- new primary key

Running sqlmigrate on the migration where the primary key changes reveals that the "id" column does not autogenerate values:

$ python manage.py sqlmigrate example 0002

BEGIN;
--
-- Add field id to example
--
ALTER TABLE "example" ADD COLUMN "id" integer NOT NULL PRIMARY KEY;
--
-- Alter field old_id on example
--
ALTER TABLE "example" ALTER COLUMN "old_id" DROP NOT NULL;

When it should be

$ python manage.py sqlmigrate example 0002

BEGIN;
--
-- Add field id to example
--
ALTER TABLE "example" ADD COLUMN "id" integer NOT NULL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY;
--
-- Alter field old_id on example
--
ALTER TABLE "example" ALTER COLUMN "old_id" DROP NOT NULL;

This seems to be related to the changes made here: https://code.djangoproject.com/ticket/30511
PR: ​https://github.com/django/django/pull/15542