Everywhere: Rename {Deprecated => Byte}String

This commit un-deprecates DeprecatedString, and repurposes it as a byte
string.
As the null state has already been removed, there are no other
particularly hairy blockers in repurposing this type as a byte string
(what it _really_ is).

This commit is auto-generated:
  $ xs=$(ack -l \bDeprecatedString\b\|deprecated_string AK Userland \
    Meta Ports Ladybird Tests Kernel)
  $ perl -pie 's/\bDeprecatedString\b/ByteString/g;
    s/deprecated_string/byte_string/g' $xs
  $ clang-format --style=file -i \
    $(git diff --name-only | grep \.cpp\|\.h)
  $ gn format $(git ls-files '*.gn' '*.gni')

Userland/Libraries/LibGUI/ModelEditingDelegate.h

@@ -97,7 +97,7 @@ public:
     {
         auto& textbox = static_cast<TextBox&>(*widget());
         if (value.is_valid())
-            textbox.set_text(value.to_deprecated_string());
+            textbox.set_text(value.to_byte_string());
         else
             textbox.clear();
         if (selection_behavior == SelectionBehavior::SelectAll)