Refs #29600 -- Removed usage of django.utils.datetime_safe in migrations.

The only effect would be if items in Atom feeds had a published date
year of < 1000 (ensuring those years are padded with leading zeros).

Removed in 244cc401.

The issue was fixed as a side effect of implementing RemoveField's reduction
of DeleteModel to a DeleteModel in ad82900a.

The PostgreSQL concat() function handles nulls and non-text values better than
the || operator.

It isn't required anymore since AlterTogetherOperations can be reduced into
CreateModels which can reduce DeleteField operations.

Thanks Akshesh Doshi for the initial patch.

This should alleviate the side effects of disabling the AlterFooOperation
reduction with RemoveField to fix refs #28862 during migration squashing
because CreateModel can perform a reduction with RemoveField.

Thanks Nick Pope for the review.

AlterFooTogether operations cannot be swapped with RemoveField operations on
the same model as they could be removing the the same field as well.

Since AlterFooTogether operations don't track what their previous value was,
it's impossible to determine whether or not the optimization is safe so the
only way to proceed is to disable the optimization.

Thanks Ramiro Morales for the in-depth analysis of the issue.

Refs #24828

Thanks Sassan Haradji for the report.