Decide on django/pulp versions for galaxy_ng 4.3

We need to address https://pulp.plan.io/issues/8691 "Pulp fails against django 2.2.21"

Current options for 4.3 code freeze:

• wait on fix to occur in django, likely django==2.2.23
• temporarily pin to django==2.2.20 (mostly to get CI stable, and unjam any stable-4.3 prs)
• wait on https://pulp.plan.io/issues/8691 to be resolved in pulpcore 3.11

Currently it seems like fix for  https://pulp.plan.io/issues/8691 is going to be dependent on the upstream django fix.

Additionally, it appears that there will be a django-2.2.22 today that fixes an unrelated CVE, but presumably something we want in stable-4.3.