Remove deprecated DCS_SESSION_COOKIE_SAMESITE settings

[Chessing234]

Summary

• Replace deprecated DCS_SESSION_COOKIE_SAMESITE with Django's native SESSION_COOKIE_SAMESITE in LMS settings.
• Remove unused DCS_SESSION_COOKIE_SAMESITE_FORCE_ALL, which has had no effect since django-cookies-samesite was removed in 2021.
• Drop the production alias SESSION_COOKIE_SAMESITE = DCS_SESSION_COOKIE_SAMESITE.

Fixes #38757

Test plan

• Verify LMS starts with SESSION_COOKIE_SAMESITE = 'None' in common settings
• Verify devstack still uses 'Lax' for local HTTP development

Made with Cursor

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