Update STUN servers in circumvention settings

Removed:

• stun.l.google.com:19302
• stun.bluesip.net:3478
• stun.dus.net:3478
• stun.sonetel.net:3478
• stun.voys.nl:3478

Added:

• stun.mixvoip.com:3478
• stun.nextcloud.com:3478
• stun.bethesda.net:3478
• stun.nextcloud.com:443

I also removed a bunch of servers from the tm settings because not all of them support NAT behaviour discovery on the alternate port.

Related to tpo/anti-censorship/pluggable-transports/snowflake#40304 (closed)