Update Django 4.2 LTS version to 4.2.20 (#2592)

This solves GitHub's Dependabot security alerts. The alerts are created because we still use an older Django version in requirements.txt when testing with older Python versions. But there is no security impact, because Django is only used in CI and not exposed as a web service.

Author: intgr

requirements.txt

@@ -7,7 +7,7 @@ pytest-xdist==3.6.1
 
 # Django deps:
 psycopg2-binary
-Django==4.2.16; python_version < '3.10'
+Django==4.2.20; python_version < '3.10'
 Django==5.1.7; python_version >= '3.10'
 -e ./ext
 -e .[redis,compatible-mypy,oracle]