Add More STUN Servers to Snowflake

We have recently removed 2 stun servers from snowflake bridge line as they are blocked or inaccessible #40523 (comment 3365425).

I have tested the remaining servers recently, and there are 2 of them currently still working and supports testing udp types:

Server	Type Discovery
stun.epygi.com:3478	Yes
stun.uls.co.za:3478	Yes
stun.nextcloud.com:3478	Yes Broken
stun.nextcloud.com:443	Yes Broken
stun.antisip.com:3478	No
stun.bethesda.net:3478	Broken Yes (do not support v2ray's nat type discovery, but should work with snowflake it is broken and detected by v2ray, will result in silent failure if not workaround-ed)

We should consider adding more stun servers that supports nat type testing to reduce load or dependency on remaining servers.