patch 8.2.2921: E704 for script local variable is not backwards compatible

Problem:    E704 for script local variable is not backwards compatible.
            (Yasuhiro Matsumoto)
Solution:   Only give the error in Vim9 script.  Also check for function-local
            variable.
diff --git a/src/version.c b/src/version.c
index 1c01adb..321047f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2921,
+/**/
     2920,
 /**/
     2919,