Support Net::DNS 0.75+

&Net::DNS::PACKETSZ has been removed from Net::DNS and was not officially part of the API. Set packet size to 512, which was default value for &Net::DNS::PACKETSZ anyway.

https://rt.cpan.org/Public/Bug/Display.html?id=96479

Older versions of Net::DNS had this comment:

Default udppacketsize is &Net::DNS::PACKETSZ (512)