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.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2921,
 /**/
     2920,
 /**/