Fix Use of a broken or weak cryptographic hashing algorithm on sensitive data

[ekuboo100]

Fix the problem, replace the use of MD5 with a strong cryptographic hash function, such as SHA-256, for hashing the session ID. This change should be made in the new_store_file method, where the hash is computed for the session ID. Specifically, replace require 'digest/md5' and Digest::MD5.hexdigest(session_id)[0,16] with require 'digest' and Digest::SHA256.hexdigest(session_id)[0,16]. This ensures that a strong hash function is used, reducing the risk of collisions and improving security. No other changes are needed, as the hash is still truncated to 16 characters for filename purposes.