AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

Userland/Libraries/LibCore/Directory.cpp

@@ -53,7 +53,7 @@ ErrorOr<Directory> Directory::adopt_fd(int fd, Optional<LexicalPath> path)
     return Directory { fd, move(path) };
 }
 
-ErrorOr<Directory> Directory::create(String path, CreateDirectories create_directories, mode_t creation_mode)
+ErrorOr<Directory> Directory::create(DeprecatedString path, CreateDirectories create_directories, mode_t creation_mode)
 {
     return create(LexicalPath { move(path) }, create_directories, creation_mode);
 }