Switch from the next argument to returning values in router guards

While learning more about some changes made in Vue Router 4 to troubleshoot #1083 (closed), I noticed that the next argument in router guards, which is used to give instructions to the router on what to do (for example redirect to a 404 page or a login page), is "removed but still supported", yet another weird way to avoid saying "deprecated".

All of the router guards should instead use the new syntax based on return, which will make writing a router guard less prone to errors.