net/traefik: Update to upstream version 2.6.0

Details:

• New 2.6.0 stable branch, details see https://github.com/traefik/traefik/releases/tag/v2.6.0
• No breaking config changes, details see https://doc.traefik.io/traefik/migration/v2/#v25-to-v26

MFH: 2022Q1