add deprecation for jwt_secret

.env.example

@@ -18,6 +18,9 @@ NEXT_PUBLIC_WEBAPP_URL=http://localhost:3000
 # Use `openssl rand -base64 32` to generate a key
 NEXTAUTH_SECRET=secret
 
+# Deprecation note: JWT_SECRET is no longer used
+# JWT_SECRET=secret
+
 POSTGRES_USER=unicorn_user
 POSTGRES_PASSWORD=magical_password
 POSTGRES_DB=calendso