Searches for $\Lambda_b^0$ and $\Xi_b^0$ decays to $K_{\mathrm{S}}^0p{\pi^{-}}$ and $K_{\mathrm{S}}^0p{K^{-}}$ final states with first observation of the $\Lambda_b^0\to K_{\mathrm{S}}^0p{\pi^{-}}$ decay