Remove unicode in Zebra's user agent

[teor2345]

Motivation

Zebra doesn't appear in node explorers like https://blockchair.com/zcash/nodes
(It also doesn't appear in DNS seeders #1904, but recent fixes might have resolved that issue.)

This might be because we use multi-byte UTF-8 in Zebra's user-agent.

Currently, accurate network metrics are more important than encouraging UTF-8 support in the Zcash ecosystem.

Specifications

User-agents are loosely specified as US-ASCII, but you have to track back through a lot of references and spec grammar to find that restriction:
https://github.com/bitcoin/bips/blob/master/bip-0014.mediawiki#proposal
https://datatracker.ietf.org/doc/html/rfc1945#section-10.15
https://datatracker.ietf.org/doc/html/rfc1945#section-3.7
https://datatracker.ietf.org/doc/html/rfc1945#section-2.2

However, it is quite likely that some block explorers do not support multi-byte UTF-8.

Solution

• Delete multi-byte UTF-8 characters from the Zebra user-agent

Review

Anyone can review this PR.

Reviewer Checklist

• Fix is implemented as described

• Optional: Manually deployed node version appears in the node list

  • We might need to configure listen_addr with the external IP address before this works
  • Deploy Job: https://github.com/ZcashFoundation/zebra/actions/runs/963230415
  • Node List: https://blockchair.com/zcash/nodes

Follow Up Work

Automatically use zebrad version for the zebra-network user agent #2375

[mpguerra]

LGTM!

[dconnolly]

😭🦓✨

[teor2345]

This hasn't impacted the node list yet:
https://blockchair.com/zcash/nodes

Maybe we need to run a zebrad node with a publicly reachable listener port. Or maybe there are other parts of the network protocol that we don't implement as expected.