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/LibIMAP/MessageHeaderEncoding.cpp

@@ -20,7 +20,7 @@ ErrorOr<ByteBuffer> decode_rfc2047_encoded_words(StringView input)
 
     while (!lexer.is_eof()) {
         auto ascii_view = lexer.consume_until("=?"sv);
-        DeprecatedString ascii = ascii_view.replace("\r"sv, " "sv, ReplaceMode::All);
+        ByteString ascii = ascii_view.replace("\r"sv, " "sv, ReplaceMode::All);
         ascii = ascii.replace("\n"sv, " "sv, ReplaceMode::All);
         TRY(output.try_append(ascii));
         if (lexer.is_eof())