By "less secure", Google means that dual-factor authentication (Oauth2) is not now enabled on the Outlook instance on your PC. Since at least Outlook 2016, Oauth2 has been built in, but it is not automatically enabled. For Outlook 2013, it is also available, but requires a Registry hack.

To enable Oauth2 in Outlook 2013:

• Press Windows, type rege, and select Regedit.
• In the Location bar at top, go to HKCU:\Software\Microsoft\Office\15.0\Common\Identity (you can paste that into the bar).
• In Identity, right-click and create a new DWORD value named EnableADAL.
• Double-click that value and set it to 1.

Then, in either version, open Outlook account settings, and try to change authentication to Oauth2, as in GMail instructions.