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/Applets/Network/main.cpp

@@ -103,7 +103,7 @@ private:
         m_connected = connected;
     }
 
-    String get_adapter_info()
+    DeprecatedString get_adapter_info()
     {
         StringBuilder adapter_info;
 
@@ -154,7 +154,7 @@ private:
         return adapter_info.to_string();
     }
 
-    String m_adapter_info;
+    DeprecatedString m_adapter_info;
     bool m_connected = false;
     bool m_notifications = true;
     NonnullRefPtr<Gfx::Bitmap> m_connected_icon;