gittuf: Policy can be rolled back to prior valid version

gittuf is a platform-agnostic Git security system. Prior to 0.14.0, an attacker with push access to gittuf's Reference State Log (RSL) can roll back the current policy to any previous policy trusted by the current set of root keys. gittuf determines the policy to load by inspecting the RSL. Except for the very first policy (which is automatically trusted given gittuf's TOFU model, or verified against manually specified keys), whenever an RSL entry that points to a new policy is encountered, gittuf validates that this policy is trusted. This is done by checking that the new policy’s root metadata is signed by the required threshold of the current policy's root keys. Because of this, an attacker with push access to the RSL may create a new entry that references an old policy (that is trusted by the most recent policy's set of root keys), thereby rolling back gittuf's policy to the attacker's chosen state. This vulnerability is fixed in 0.14.0.