Update to upstream release 2.3.1

Details:

• The Traefik v1 branch has been on maintenance mode for over a year and is no longer guaranteed to receive security updates, see: https://community.traefik.io/t/end-of-support-of-1-x/2451
• Updating of configuration files will be necessary for users who have not previously used net/traefik2 (which was introduced in 2020Q2 to allow for the eventual depreciation of the v1 branch). Instructions and a link to an automated migration tool are provided in pkg-message.