Assign some ports I maintain from Django 1.11 to Django 2.2

Django 1.11 is End-of-Life since April 2020 thus switch the following ports
to Django 2.2 (which has LTS support until April 2022):

• www/py-django-auth-ldap
• www/py-django-cacheops
• www/py-django-filter
• www/py-django-prometheus

Adjust the required Python version as Django 2.2 needs at least Python 3.5+
and bump the PORTREVISION for package change.

Mark the related py-dj22-* port variants as DEPRECATED because they're
obsolete now. Don't set an expiration date yet as they're still used by
some consumers and a few more Django 1.11 ports need to be assigned to
Django 2.2 first.