Add Django-5.2 support on 2025-04

[shimizukawa]

Django 5.2: April 2025 https://docs.djangoproject.com/en/dev/releases/5.2/

• update tox and github CI
• update pyproject.toml
• update document
• bump version x.Y.z for imcompatible update

[tolomea]

I recently upgraded a large project to Django 5.2 and did not notice that this caused django-redshift-backend to downgrade to 3.0.0 and now I'm hitting this #109 issue in prod.

[OJFord]

Is anything actually broken by using 5.2, or could the dependency just drop that constraint?