patch 8.2.4515: old subsitute syntax is still supported

Problem:    Old subsitute syntax is still supported.
Solution:   Disallow using backslash after ":s" in Vim9 script.
diff --git a/src/version.c b/src/version.c
index 238e627..4bbe08c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4515,
+/**/
     4514,
 /**/
     4513,