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.
diff --git a/src/version.c b/src/version.c
index 877d63d..ff1a97a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1788,
+/**/
     1787,
 /**/
     1786,