coturn: Possible abuse of STUN and need to turn off UDP

It is possible that adversaries are abusing STUN servers for DDoS. See https://alioth-lists.debian.net/pipermail/freedombox-discuss/2024-May/009800.html

https://www.securityweek.com/organizations-warned-stun-servers-increasingly-abused-ddos-attacks/

Some people are recommending that UDP be turned off for STUN https://www.netscout.com/blog/asert/session-traversal-utilities-nat-stun However, this means that UDP connection will no longer work for clients behind NAT leading to more expensive TCP connection.

Explore the best practices (especially from upstream Coturn project) and make changes in FreedomBox accordingly.