By Andy Bruce and William James LONDON (Reuters) -Prime Minister Boris Johnson told Britons on Thursday he was “on your side”, vowing to build a high-growth, low-tax economy where everyone could buy their own home, in his latest attempt at a reset after surviving a major revolt against his leadership. In a wide-ranging speech, the Conservative leader vowed to tackle the soaring cost of living and address long-standing problems such as unaffordable childcare. But many economists were doubtful about Johnson’s promises, with official forecasters saying tax revenue is on course to rise to the high…