Enforce email encryption: use enable_starttls not enable_starttls_auto

[eliotsykes]

For example, one notable finding involved a vulnerability in the email system, where fallback to unencrypted transmission could expose sensitive emails. The recommended fix was straightforward—replace enable_starttls_auto with enable_starttls to enforce strict TLS encryption.

Source: https://blog.rubygems.org/2024/12/11/security-audit.html