Use StrAppend instead of std::string::append

The current code doesn't compile in Chrome because std::string::append doesn't accept an absl::string_view as input and there's no implicit conversion. The new code is also more efficient.

PiperOrigin-RevId: 399871014

quic/core/http/capsule.cc

diff --git a/quic/core/http/capsule.cc b/quic/core/http/capsule.cc
index 0f9eefc..8f72955 100644
--- a/quic/core/http/capsule.cc
+++ b/quic/core/http/capsule.cc
@@ -436,7 +436,7 @@
   if (parsing_error_occurred_) {
     return false;
   }
-  buffered_data_.append(capsule_fragment);
+  absl::StrAppend(&buffered_data_, capsule_fragment);
   while (true) {
     const size_t buffered_data_read = AttemptParseCapsule();
     if (parsing_error_occurred_) {