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/Applications/Welcome/WelcomeWidget.h

@@ -33,5 +33,5 @@ private:
     RefPtr<WebView::OutOfProcessWebView> m_web_view;
 
     size_t m_initial_tip_index { 0 };
-    Vector<String> m_tips;
+    Vector<DeprecatedString> m_tips;
 };