Prepare to deprecate gfe2_reloadable_flag_send_quic_fallback_server_config_on_leto_error

This flag has been manually enabled in piccolo, so the normal feature-flags process is no longer driving it forward.  This CL marks it rolled-out manually.

PiperOrigin-RevId: 360509340
Change-Id: I727692892239a81bc36507864b9c096c381840fc

quic/core/quic_flags_list.h

diff --git a/quic/core/quic_flags_list.h b/quic/core/quic_flags_list.h
index 19a466f..708ae5c 100644
--- a/quic/core/quic_flags_list.h
+++ b/quic/core/quic_flags_list.h
@@ -66,7 +66,7 @@
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_update_packet_content_returns_connected, true)
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_use_encryption_level_context, true)
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_use_write_or_buffer_data_at_level, false)
-QUIC_FLAG(FLAGS_quic_reloadable_flag_send_quic_fallback_server_config_on_leto_error, false)
+QUIC_FLAG(FLAGS_quic_reloadable_flag_send_quic_fallback_server_config_on_leto_error, true)
 QUIC_FLAG(FLAGS_quic_restart_flag_dont_fetch_quic_private_keys_from_leto, false)
 QUIC_FLAG(FLAGS_quic_restart_flag_quic_dispatcher_support_multiple_cid_per_connection_v2, true)
 QUIC_FLAG(FLAGS_quic_restart_flag_quic_offload_pacing_to_usps2, false)