Switch to using an external Etherpad role

This new role also adds native Traefik support and support for other
(non-`amd64`) architectures via self-building.

playbooks/matrix.yml

@@ -103,7 +103,8 @@
     - custom/matrix-ldap-registration-proxy
     - custom/matrix-ma1sd
     - custom/matrix-dimension
-    - custom/matrix-etherpad
+    - galaxy/etherpad
+    - custom/etherpad-proxy-connect
     - custom/matrix-email2matrix
     - custom/matrix-sygnal
     - galaxy/ntfy