Deprecate `registerCandidate` method of native NeoToken

[AnnaShaleva]

Summary or problem description
Starting from #3597 the preferable way of candidate registration is a GAS transfer to native NeoToken contract. The original way of registration via registerCandidate method call supposed to be deprecated and eventually removed from the native NeoToken manifest.

Do you have any solution you want to propose?
Schedule removal of registerCandidate method to Gorgon hardfork (Faun is too close, we need to give some transition period to dApps to migrate to the new way of candidate registration). Set ActiveTill of registerCandidate method to Gorgon. Make an announcement of this change.

Where in the software does this update applies to?

• Native contracts