feat!: Upgrade backend (Python, Django, Wagtail)

[mgax]

⚠️ After deploying this change, the following command should be run on the production instance, to copy daily search hits data from the wagtailsearch.Query model to wagtailsearchpromotions.Query:

./manage.py copy_daily_hits_from_wagtailsearch

---

Upgrade the backend across the board:

• ~Fix deprecation warnings~ (moved to https://github.com/ietf-tools/wagtail_website/pull/352)
  • [x] Move bibliography app config to apps.py
  • [x] Replace url with re_path
  • [x] Replace usage of deprecated assertEquals
  • [x] Treat warnings as testing errors
• [x] Switch to pip-tools version pinning
• [x] Upgrade to Django 4.0
  • ~Add workaround for missing source maps (needs further investigation)~ Fix source map output in webpack
• [x] Drop django-compressor and django-libsass (they don't seem to do anything useful; the frontend build already compiles *.scss files)
• [x] Upgrade to Wagtail 4.2
• [x] Upgrade to Django 4.1
• [x] Upgrade to Python 3.11
• [x] Write test for a simple search query (because search broke when upgrading to Wagtail 5.0)
• [x] Upgrade to Wagtail 5.0
  • [x] Update the search views to match the current reference implementation.
  • [x] Remove partial_match for index fields. Search is using the database backend which doesn't support partial matches anyway.
  • [x] Check if slugs are still being generated in the admin.
• [x] Django 4.2
• [x] Wagtail 5.1 (fixes https://github.com/ietf-tools/wagtail_website/issues/322)
• [x] Wagtail 5.2
• [x] Python 3.12
• [x] Other Python dependencies
• ~Frontend upgrades~ (moved to https://github.com/ietf-tools/wagtail_website/pull/354)
  • Check for unmaintained frontend dependencies
  • Node 18
  • Node 20