Replace absl::optional with std::optional in QUICHE.

Chrome has switched to those two types being the same, so we don't have any reason to use the Abseil version anymore.

PiperOrigin-RevId: 580345251
diff --git a/quiche/quic/core/quic_connection_test.cc b/quiche/quic/core/quic_connection_test.cc
index b7f6e1a..ecfb82b 100644
--- a/quiche/quic/core/quic_connection_test.cc
+++ b/quiche/quic/core/quic_connection_test.cc
@@ -17126,7 +17126,7 @@
               GenerateNextConnectionId(connection_id_))
       .WillOnce(Return(server_issued_cid_for_preferred_address));
   EXPECT_CALL(visitor_, MaybeReserveConnectionId(_)).WillOnce(Return(true));
-  absl::optional<QuicNewConnectionIdFrame> frame =
+  std::optional<QuicNewConnectionIdFrame> frame =
       connection_.MaybeIssueNewConnectionIdForPreferredAddress();
   ASSERT_TRUE(frame.has_value());
 
@@ -17179,7 +17179,7 @@
               GenerateNextConnectionId(connection_id_))
       .WillOnce(Return(server_issued_cid_for_preferred_address));
   EXPECT_CALL(visitor_, MaybeReserveConnectionId(_)).WillOnce(Return(true));
-  absl::optional<QuicNewConnectionIdFrame> frame =
+  std::optional<QuicNewConnectionIdFrame> frame =
       connection_.MaybeIssueNewConnectionIdForPreferredAddress();
   ASSERT_TRUE(frame.has_value());