Add 'NegativeUNL' as a supported amendment:

The Negative UNL is a feature of the XRP Ledger consensus protocol that
improves liveness (the network's ability to make forward progress) during
a partial outage. Using the Negative UNL, servers adjust their effective
UNLs based on which validators are currently online and operational, so
that a new ledger version can be declared validated even if several trusted
validators are offline.

The Negative UNL has no impact on how the network processes transactions
or what transactions' outcomes are, except that it improves the network's
ability to declare outcomes final during some types of partial outages.

The feature was originally introduced with version **1.6.0** but it was
only possible to manually enable this. If merged, this commit introduces
the amendment associated with the feature so that server operators can
vote on whether to enable this feature.

For more details, please see https://xrpl.org/negative-unl.html

This commit closes #3898.

src/ripple/protocol/impl/Feature.cpp

@@ -131,7 +131,7 @@ detail::supportedAmendments()
         "fix1781",
         "HardenedValidations",
         "fixAmendmentMajorityCalc",
-        //"NegativeUNL",      // Commented out to prevent automatic enablement
+        "NegativeUNL",
         "TicketBatch",
         "FlowSortStrands",
         "fixSTAmountCanonicalize",