Guide/django+database instruction deprecation

[MVKozlov]

Starting from /guides/django/200_real_apps/040_db.html, guide raise errors if used with recent versions of images.
At first, mysqlclient for pip just doesn't build. Here they write that mariadb-dev should be added to packages. With this addition it compiles successfully.

But this is not enough. at the stage of starting the migration job, we get
django.db.utils.NotSupportedError: MySQL 8 or later is required (found 5.7.42).
so the db image version needs to be raised from 5.7 to 8.0 (in the topic about secrets, this has already been done)

Maybe this, of course, is deliberately left like that? :)