docs: update middleware.ts references to proxy.ts for Next.js 16 (#13373)

Next.js 16 renamed middleware.ts to proxy.ts and the exported function
from middleware to proxy. This updates all documentation references
across installation, session protection, edge compatibility, migration,
RBAC, passkey, and Prisma adapter pages to reflect the new convention.

Co-authored-by: FGameIn <fares.galal@gamein.ae>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>