By Kate Holton LONDON (Reuters) -Senior members of Boris Johnson’s Conservative Party urged challengers to the prime minister to rein in their ambitions and focus instead on steering Britain through the biggest plunge in living standards in a generation. Johnson is battling for his political career amid a crisis which has gripped his government over three months, with a growing number of lawmakers calling publicly for a change at the top to rebuild trust with voters. Johnson has apologised after he and staff held parties during strict coronavirus lockdowns, at a time when many people could not…