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/LibIPC/Encoder.h

@@ -43,7 +43,7 @@ public:
     Encoder& operator<<(double);
     Encoder& operator<<(char const*);
     Encoder& operator<<(StringView);
-    Encoder& operator<<(String const&);
+    Encoder& operator<<(DeprecatedString const&);
     Encoder& operator<<(ByteBuffer const&);
     Encoder& operator<<(JsonValue const&);
     Encoder& operator<<(URL const&);