Update list of Snowflake STUN servers in default bridge line

As described in tpo/anti-censorship/pluggable-transports/snowflake#40241 (closed) and the anti-censorship mailing list thread we are removing stun.stunprotocol.org from the set of STUN servers in our default settings and reserving it for use only when needed. We are also removing stun.altar.com.pl because it is seemingly no longer online.