Implement UpdatePolicy() API (aka UpdatableAdapter interface)

[hsluoyz]

See the video at: https://www.bilibili.com/video/BV13r4y1M7AC , 16:10

UpdatePolicy() is introduced by this PR: apache/casbin#592

[hsluoyz]

Released v3.0.6: https://github.com/casbin/gorm-adapter/releases/tag/v3.0.6