Summary of Changes

This PR removes the authentication code {CODE} from the email subject line.

What problem am I trying to solve?
The email subject line is significantly less protected than the content:
It's often displayed in plain text, for example, on smartphone lock screens, in email notifications, or previews.
This makes it much easier for others to see the subject line without opening the email (shared inbox, office, support).
Forwarding and auto-replies also pose a problem; some systems only use the subject line and reply with it again, unintentionally including the code in other emails.

Testing Instructions

Activate the MFA plugin "Multi-factor Authentication - Authentication Code by Email" and configure it for a user.
Use this login method for the user. An email will arrive in the inbox.

Actual result BEFORE applying this Pull Request

Subject e.g.: "Your 610-alpha2 authentication code is -123456-"

Expected result AFTER applying this Pull Request

Subject e.g.: "Your 610-alpha2 authentication code"
And the code is now only contained in the email body.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed