After NU5 mainnet activation, set the minimum mainnet network protocol version to NU5

[teor2345]

Motivation

There might only be a small number of Zebra instances running when NU5 activates on mainnet.

So instead of automatically closing connections to outdated peers, we can just set the mainnet minimum protocol version to NU5 after NU5 mainnet activation.

Then we can upgrade and restart all the running Zebra instances, and they will reject outdated peers as soon as they start up, regardless of the synced block height.

Specifications

Original specification: ZIP 201: Network Peer Management for Overwinter

Since this is a network protocol specification, Zebra is free to adopt any compatible design.

Designs

Draft Zebra behaviour: ZIP 252: Deployment of the NU5 Network Upgrade - Support in Zebra

Related Work

This ticket is a smaller temporary substitute for #2262 and #2451.

If we make a similar minimum version change on mainnet, we don't need to implement #2451 at all.
But we still need to implement #2262 for the next network upgrade.

[teor2345]

This is a temporary substitute for #2262.

We should try to implement #2262 before NU5 mainnet activation, because we're going to have to do it eventually anyway.