Replaces direct encrypter calls with decrypt_value() and encrypt_value()
helpers throughout the codebase for consistent error handling:
- Config controller: SMTP, SMS, Mailchimp credential encryption
- Email_lib: SMTP password decryption
- Sms_lib: SMS password decryption
- Mailchimp_lib: API key decryption
- Customers controller: Mailchimp list ID decryption
Removes nullable EncrypterInterface property from Config controller as
encryption is now handled via helper functions.
GitHub-Issue: #4554