patch 8.2.2449: Vim9: flatten() always changes the list type

Problem:    Vim9: flatten() always changes the list type.
Solution:   Disallow using flatten() and add flattennew().
diff --git a/src/version.c b/src/version.c
index 5a89933..d34839c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2449,
+/**/
     2448,
 /**/
     2447,