net/traefik: Update to upstream version 2.5.1
Details:
- Major upgrade to the 2.5.x branch: https://github.com/traefik/traefik/releases/tag/v2.5.1
- Almost no breaking changes regarding config files. For a small number of users, tweaks might be necessary, see: https://doc.traefik.io/traefik/migration/v2/#v24-to-v25