Increases default script thresholds

Fixes #4424 with companion pro commit

scripts/plugins/alerts/security/syn_scan_alert/user_scripts/host/syn_scan_attacker.lua

@@ -19,9 +19,8 @@ local script = {
   -- The default threshold value. The format is specific of the
   -- "threshold_cross" input builder
   default_value = {
-    -- "> 50"
     operator = "gt",
-    threshold = 50,
+    threshold = 256,
   },
 
   -- See below

scripts/plugins/alerts/security/syn_scan_alert/user_scripts/host/syn_scan_victim.lua

@@ -21,7 +21,7 @@ local script = {
   default_value = {
     -- "> 30"
     operator = "gt",
-    threshold = 30,
+    threshold = 256,
   },
 
   -- See below

scripts/plugins/alerts/security/syn_scan_alert/user_scripts/network/syn_scan_victim.lua

@@ -16,8 +16,10 @@ local script = {
   -- This script is only for alerts generation
   is_alert = true,
 
-  -- No default threshold configuration is provided
-  default_value = {},
+  default_value = {
+     operator = "gt",
+     threshold = 32768,
+  },
 
   -- See below
   hooks = {},