Prepare to deprecate gfe2_restart_flag_dont_fetch_quic_private_keys_from_leto

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: 393780804
diff --git a/quic/core/quic_flags_list.h b/quic/core/quic_flags_list.h
index b2940bb..2a77060 100644
--- a/quic/core/quic_flags_list.h
+++ b/quic/core/quic_flags_list.h
@@ -6,7 +6,7 @@
 
 #ifdef QUIC_FLAG
 
-QUIC_FLAG(FLAGS_quic_restart_flag_dont_fetch_quic_private_keys_from_leto, false)
+QUIC_FLAG(FLAGS_quic_restart_flag_dont_fetch_quic_private_keys_from_leto, true)
 
 QUIC_FLAG(FLAGS_quic_restart_flag_quic_offload_pacing_to_usps2, false)
 // A testonly reloadable flag that will always default to false.