Fixed #23738 -- Allowed migrating from NULL to NOT NULL with the same default value

Thanks to Andrey Antukh for the report.
Added a warning about direct settings manipulation in tests.

Thanks Thomas Chaumeny for the initial patch.
Fixed #23732 -- Corrected and enhanced select_related() docs.

Thanks Daniele Procida for the report and review.
Renamed SpatiaLite feature flag introduced in refs #23152.

Thanks Doug Goldstein for the suggestion.
Fixed #23468 -- Added checks for duplicate fixtures directories in loaddata.

If settings.FIXTURE_DIRS contains duplicates or a default fixture
directory (app_name/fixtures), ImproperlyConfigured is raised.

Thanks to Berker Peksag and Tim Graham for review.
Fixed #23731 -- Fixed migrations crash when adding blank GeometryFields on PostGIS.

Thanks raratiru for the report.