patch 8.2.3423: Vim9: list += list creates a new list in :def function

Problem:    Vim9: list += list creates a new list in :def function.
Solution:   Append to the existing list.
diff --git a/src/version.c b/src/version.c
index 8cd295c..9f4ea76 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3423,
+/**/
     3422,
 /**/
     3421,