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/Utilities/arp.cpp

@@ -5,11 +5,11 @@
  */
 
 #include <AK/Assertions.h>
+#include <AK/DeprecatedString.h>
 #include <AK/IPv4Address.h>
 #include <AK/JsonObject.h>
 #include <AK/MACAddress.h>
 #include <AK/QuickSort.h>
-#include <AK/String.h>
 #include <AK/Types.h>
 #include <LibCore/ArgsParser.h>
 #include <LibCore/Stream.h>
@@ -53,10 +53,10 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     };
 
     struct Column {
-        String title;
+        DeprecatedString title;
         Alignment alignment { Alignment::Left };
         int width { 0 };
-        String buffer;
+        DeprecatedString buffer;
     };
 
     Vector<Column> columns;