install/upgrade/5.8.x-to-6.0.0.md: now 6.0.0 is stable

docs/install/upgrade/5.8.x-to-6.0.0.md

@@ -1,4 +1,4 @@
-# Upgrade Kamailio from v5.8.x to v6.0.0 (devel)
+# Upgrade Kamailio from v5.8.x to v6.0.0 (stable)
 
 The page contains the details about the changes that were made to old
 components during the development of v6.0.0 compared with what existed