patch 8.2.1756: Vim9: :let will soon be disallowed

Problem:    Vim9: :let will soon be disallowed.
Solution:   Add v:disallow_let temporarily.  Fix tests.
diff --git a/src/version.c b/src/version.c
index 2072b35..a0bea6f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1756,
+/**/
     1755,
 /**/
     1754,