Refactor TURN/STUN config:

- Removed ?transport=udp from Nextcloud Talk TURN server definitions
- Dropped --no-tcp-relay to allow TCP fallback
- Removed invalid UDP mapping on TLS port
- Introduced switch between REST secret auth and lt-cred-mech via COTURN_USER_AUTH_ENABLED
- Added user_auth_enabled flag in coturn config for flexibility

See: https://chatgpt.com/share/68d7d601-3558-800f-bc84-00d7e8fc3243

roles/web-app-bigbluebutton/templates/docker-compose.override.yml.j2

@@ -21,7 +21,6 @@ services:
       --fingerprint
       --no-multicast-peers
       --no-cli
-      --no-tcp-relay
       --min-port={{ BBB_RELAY_PORT_START }}
       --max-port={{ BBB_RELAY_PORT_END }}
       --external-ip=${EXTERNAL_IPv4}