add(network-params): Allow configuring NU6 activation height on Regtest

[arya2]

Motivation

This is useful for testing changes related to NU6 deployment.

Solution

Adds an argument to Network::new_regtest() and testnet::Parameters::new_regtest() for NU6 activation height.

Tests

This will be tested in #8694.

Follow-up Work

Update the Zebra book (blocked on #8694).

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.