Add some DNS hostnames to securedns.txt

[touhidurrr]

Sources:

1. DNSFilter: https://help.dnsfilter.com/hc/en-us/articles/1500008110762-Enable-DNS-over-TLS
2. Yandex DNS: https://dns.yandex.com/
3. UncensoredDNS: https://blog.uncensoreddns.org/
4. IPv6DNS: https://www.ipv6dns.com/pconfig_en/
5. CONTROL D: https://controld.com/free-dns
6. SafeServe by namecheap: https://www.namecheap.com/dns/free-public-dns/#how