refactor: remove server actions env var, keep only allowedDevOrigins
Based on feedback, removing NEXT_SERVER_ACTIONS_ALLOWED_ORIGINS as it's just a shorthand for existing config. Keeping only NEXT_ALLOWED_DEV_ORIGINS which is the main requirement for sandboxed environments.
- Remove NEXT_SERVER_ACTIONS_ALLOWED_ORIGINS env var parsing
- Remove related tests for server actions env var
- Remove server actions env var documentation
- Keep only NEXT_ALLOWED_DEV_ORIGINS functionality