Activate new slashing feature flag

What

Enable feature flag guarding the new 'adaptive' slashing mechanism.

Why

Because you can't stop progress!

FYI: The feature is described in https://docs.google.com/document/d/163AXCgKyrVWeLlNAoKDbsGIzQpwe-iTIRIws5dgxXQ0/edit#heading=h.vb9j318ygdzd

How

By replacing alpha occurences of ns_enable = false to ns_enable = true

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR