By Andrew MacAskill LONDON (Reuters) – British Prime Minister Boris Johnson should be replaced immediately rather than be allowed to remain as a caretaker leader until his successor is found, several Conservative members of parliament said on Thursday. Johnson will announce his resignation as prime minister on Thursday, a government source said, after he was abandoned by ministers and his Conservative Party’s lawmakers who said he was no longer fit to govern. Five Conservative member of parliament demanded he resign on Thursday and said another minister should take over during a process that c…