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.

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3423,
 /**/
     3422,
 /**/