Search for heavy resonances decaying into a pair of Z bosons in the ℓ⁺ℓ^-ℓ^{′ +}ℓ^{′ -} and ℓ⁺ℓ^-νν¯ final states using 139 fb ^{- 1} of proton–proton collisions at √s=13 TeV with the ATLAS detector