Add Mullvad as default DNS option

[shuvashish76]

Request to add mullvad DNS to our default preset list for users.
Following Blocklists are used: https://github.com/mullvad/dns-blocklists#lists
Reason : AFAIK this is the only audited DNS that I'm aware of.

I think Extended one is good enough for most users.
Add these two :

• IPv4 address : 194.242.2.5 DoT port : 853
• IPv6 address : 2a07:e340::5 DoT port : 853