update to kronosnet v1.21 stable release

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

diff --git a/upstream b/upstream
index b8d18c8360fd39cb04748e8bc1ee26de4afa4cbd..f66f436bbed67bdccd2c6ba2886bcbab9d1c4ca8 160000 (submodule)
--- a/upstream
+++ b/upstream
@@ -1 +1 @@
-Subproject commit b8d18c8360fd39cb04748e8bc1ee26de4afa4cbd
+Subproject commit f66f436bbed67bdccd2c6ba2886bcbab9d1c4ca8