patch 8.2.2305: Vim9: "++var" and "--var" are silently accepted

Problem:    Vim9: "++var" and "--var" are silently accepted.
Solution:   Give an error message.
diff --git a/src/version.c b/src/version.c
index 77f032d..0e711ed 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2305,
+/**/
     2304,
 /**/
     2303,