You can customise the text from your /admin/customize/site_texts page, and the string you’re looking for is js.post.notice.new_user. 
As a general tip, you can type a key part of the text you’re looking to replace into the search box on that page and it should bring up the one you want. 
There is also the returning user post notice, though I don’t think that one sounds like it will be as sensitive - but it’s js.post.notice.returning_user if you need it. And you can adjust what trust levels can see them using the new user notice tl and returning user notice tl admin settings:
