[atlas] Minor change to Atlas anchor DNS resolver configuration

Dear colleagues,

Each RIPE Atlas anchor runs a DNS resolver. This resolver is
configured as the primary resolver for the anchor, with the following
directive at the top of the resolver configuration (/etc/resolv.conf):

nameserver 127.0.0.1

For ease of configuration management, we are changing this directive to:

nameserver ::1

This change will not affect DNS resolution on the anchors in any way.
However, any measurement that does DNS lookups using the anchor's
primary resolver will contain "::1" instead of "127.0.0.1" in "src_addr"
field of results. If you're using any code to parse such results, and
specifically looking for "127.0.0.1", you'll need to adjust your code to
also look for "::1".

Regards,
Anand Buddhdev
RIPE NCC