net/dns: Increase UDP response buffer to 1232 bytes

[AaronFriel]

This resolves #51127 in the near term by defaulting to a larger buffer
size. This is not a permanent fix or implementation of EDNS(0) or IETF
RFC6891 (https://datatracker.ietf.org/doc/html/rfc6891).

These changes should be reviewed by someone with more experience than I.

:)

Fixes #51127
^ hotfix only.

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This PR is being closed because golang.org/cl/384076 has been abandoned.

Thanks. Let's use CL 385035 instead.