Migrate to Traefik v2

• Traefik v1 → Traefik v2
• Migrate labels
• Clean Compose files : syntax, formatting, version 3.7.
• Remove RAM / CPU limit for Etherpad.

See : https://wiki.picasoft.net/doku.php?id=technique:adminsys:migration-traefik-v2

merged

mentioned in commit f010fae1

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