Update Django to version 4.2 LTS

[timobrembeck]

Motivation

Django 4.2 LTS provides a few improvements, e.g.:

• Database optimizations (psycopg3 instead of psycopg2)
• Mitigation for the BREACH attack, so we can enable gzip in entire CMS without the workaround

for the full list, see releases notes

Proposed Solution

• Check whether we're affected by any backward incompatible changes
• Check whether all our dependencies support Django 4.2

Alternatives

The current version 3.2 LTS will be supported until April 2024, so we can delay updating for about a year.

[svenseeberg]

End of Life for 3.2 is end of Q1/2024, see https://www.djangoproject.com/download/