By Andrea Shalal and Andreas Rinke SCHLOSS ELMAU, Germany (Reuters) -U.S. President Joe Biden told allies “we have to stay together” against Russia on Sunday as G7 leaders gathered for a summit set to be dominated by war in Ukraine and its impact on food and energy supplies and the global economy. At the start of the meeting in the Bavarian Alps, four of the Group of Seven rich nations moved to ban imports of Russian gold to tighten the sanctions squeeze on Moscow and cut off its means of financing the invasion of Ukraine. However, it was not immediately clear whether there was consensus on th…