[6.1] remove authentication code subject

[tecpromotion]

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

[Copilot]

Pull request overview

This PR enhances security by removing the authentication code from the email subject line of multi-factor authentication emails, addressing concerns about sensitive information exposure through email previews, lock screen notifications, and shared inboxes.

• Removes the {CODE} placeholder from the email subject translation string
• Keeps the authentication code in the email body where it remains protected

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[Fedik]

Having the code in the subject allows to read the code without opening email. That actually is very useful on mobile.

I would suggest:

• OR change authentication to auth to make it shorter :)
• OR even better. Move the code up front: 123456 is your FoobarSiteName authentication code