AK: Stop using DeprecatedString in Base64 encoding

Author: https://github.com/gmta
Commit: 25f2e4981c
Pull-request: https://github.com/SerenityOS/serenity/pull/16580
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/Hendiadyoin1

Userland/Libraries/LibHTTP/HttpRequest.cpp

@@ -242,7 +242,9 @@ Optional<HttpRequest::Header> HttpRequest::get_http_basic_authentication_header(
     builder.append(url.username());
     builder.append(':');
     builder.append(url.password());
-    auto token = encode_base64(builder.to_deprecated_string().bytes());
+
+    // FIXME: change to TRY() and make method fallible
+    auto token = MUST(encode_base64(MUST(builder.to_string()).bytes()));
     builder.clear();
     builder.append("Basic "sv);
     builder.append(token);