Search for heavy resonances decaying into a pair of Z bosons in the $$ell ^+ell ^-ell '^+ell '^-$$ and $$ell ^+ell ^- u {{ar{ u }}}$$ final states using 139 $$mathrm {fb}^{-1}$$ of proton–proton collisions at $$sqrt{s} = 13,$$TeV with the ATLAS detector