By Michelle Nichols, Nidal al-Mughrabi and Dan Williams UNITED NATIONS/CAIRO/JERUSALEM (Reuters) – The U.N. Security Council approved a toned-down bid to boost humanitarian aid to Gaza on Friday that stopped short of a call for a ceasefire, hours after Israel signalled it was widening its ground offensive in the Palestinian enclave. The U.S., which is Israel’s main ally and had threatened to veto the Security Council motion during days of wrangling, chose instead to abstain after language was changed on hostilities and monitoring aid, a move that let the vote go through. Washington has regular…