resolve: remove server 'large' level

resolve: remove server 'large' level

This removes the DNS_SERVER_FEATURE_LEVEL_LARGE, and sets the EDNS0
advertised max packet size as if always in 'large' mode.

Without this, we always send out EDNS0 opts that limit response sizes
to 512 bytes, thus the remote server will never send anything larger
and will always truncate responses larger than 512 bytes, forcing us
to drop from EDNS0 down to TCP, even though one of the primary benefits
of EDNS0 is larger packet sizes.

Fixes: #20993