Add richer authentication support for POP3

[agungor]

You can still use an App Password to authenticate with Gmail via POP3. Here is a recent relevant topic:

Connection to the POP3 server is failing with an authentication error. Please check your POP3 settings

I recommend turning on 2FA on the account and setting up an App Password for Discourse. The policy change is about the “allow less secure app access” option being deprecated. More info on this here: