Added #permissions= as deprecated to maintain backward compatibility of Hydra::AdminPolicy with hydra-head < 6.4.
(In hydra-head < 6.4 Hydra::AdminPolicy included Hydra::ModelMixins::RightsMetadata, which provided #permissions=.)

Removed permissions= from Hydra::ModelMixins::RightsMetadata as it is now provided by included module Hydra::AccessControls::Permissions.