Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibCore/Group.cpp

@@ -45,7 +45,7 @@ ErrorOr<DeprecatedString> Group::generate_group_file() const
     if (errno)
         return Error::from_errno(errno);
 
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 
 ErrorOr<void> Group::sync()