doc: Remove downgrading warning in release notes, per 0.18 branch
doc: Remove downgrading warning in release notes, per 0.18 branch

Former-commit-id: 8a2978d784fa84250665772dd6095140e65c4244
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin