patch 8.2.1788: Vim9: still allows :let for declarations

Problem:    Vim9: still allows :let for declarations.
Solution:   Make the default for v:disallow_let one.  It can still be set to
            zero to allow for using :let.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1788,
 /**/
     1787,
 /**/