perf: fit ~20% more data into each utp packet

See a discussion of how high the `max_packet_size` could go:
https://github.com/ethereum/trin/pull/767#discussion_r1220177297

Here is some discussion internal to discv5, with an example of calculating a max packet size:
https://github.com/sigp/discv5/blob/d2e30e04ee62418b9e57278cee907c02b99d5bd1/src/service.rs#L1054-L1067

Another fairly reasonable approach to this would be to set the packet size too high on a local build, and look out for this warning log:
https://github.com/sigp/discv5/blob/d2e30e04ee62418b9e57278cee907c02b99d5bd1/src/socket/send.rs#L67-L68

Then we can reduce the max size to not trigger that warning.

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All of this work will probably have a limited lifetime, since we plan to switch to use this at some point: https://github.com/ethereum/devp2p/issues/229

Start by locating the current max_packet_size setting and read the linked trin discussion, discv5 calculation, and send.rs warning. Test a higher local packet size, then reduce it until the warning no longer appears; done means fitting more data without triggering that warning, while noting the planned future switch described in the issue.

Written by the indexing model from the issue text.