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/LibUnicode/Normalize.cpp

@@ -296,7 +296,7 @@ static Vector<u32> normalize_implementation(Utf8View string, NormalizationForm f
     VERIFY_NOT_REACHED();
 }
 
-String normalize(StringView string, NormalizationForm form)
+DeprecatedString normalize(StringView string, NormalizationForm form)
 {
     Utf8View const view { string };