docs: drop stale FEATURES-dict references from comments and docstrings

These flags are now flat Django settings, but several comments/docstrings still described
them as FEATURES-dict keys, and each kept a 'settings.FEATURES' reference alive in a
repo-wide grep. Update them to the flat setting they now describe:
- embargo/middleware.py: settings.FEATURES['EMBARGO'] -> settings.EMBARGO
- git_export.py: FEATURE['ENABLE_EXPORT_GIT'] -> ENABLE_EXPORT_GIT setting
- auto_auth.py: settings.FEATURES['AUTOMATIC_AUTH_FOR_TESTING'] -> settings.AUTOMATIC_AUTH_FOR_TESTING
- split_mongo/split.py: reword the historical entrance-exams comment off the dict syntax
- test_auto_auth.py: update the setUp comments to name the flat setting

No code/behavior change; comment/docstring text only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>