CockroachDB: When supported, resolve "ALTER COLUMN TYPE is not supported inside a transaction"

[gnat]

This is also a current issue in the Django CockroachDB driver, for migrations.

When the following issue is resolved: https://github.com/cockroachdb/cockroach/issues/49351

Inside: piccolo/apps/migrations/auto/migration_manager.py We can refactor out the special case for ALTER COLUMN at if engine.engine_type != "cockroach"