SAN FRANCISCO — Elon Musk attempted to soothe leery Twitter advertisers Thursday, a day before a deadline to close out on his $44 billion acquisition of the social media platform, saying that he is buying the platform to “help humanity” and doesn’t want it to become a “free-for-all hellscape.” The message reflects concerns among advertisers — Twitter’s chief source of revenue — that Musk’s plans to promote free speech by cutting back on moderating content will open the floodgates to more online toxicity and drive away users. “The reason I acquired Twitter is because it is important to the future…