Add support for using indexes in update() for ArrayFields.

[Mariusz Felisiak]

Django supports index lookup for ArrayFields (see ​docs). I think we should support them also in update(), e.g.

Foo.objects.update(bar_array_field__0='random text')
Foo.objects.update(bar_array_field__0=F('other_related_field'))

I know that 3rd party packages already support this e.g. ​django_postgres_extensions, but IMO we should add this to Django.

[Carlton Gibson]

OK. Seems reasonable.

[Tim Graham]

See also #24709 for using QuerySet.update() without indexes.

[Nasir Hussain]

Created a ​PR.