patch 8.2.3347: check for legacy script is incomplete

Problem:    Check for legacy script is incomplete. (Naohiro Ono)
Solution:   Also check the :legacy modifier.  Use for string concatenation
            with "." and others (issue #8756)
diff --git a/src/typval.c b/src/typval.c
index cdb6175..5fb98eb 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -1704,7 +1704,7 @@
 	int	    want_string UNUSED)
 {
     int		len;
-    int		skip_quotes = current_sctx.sc_version >= 4 || in_vim9script();
+    int		skip_quotes = !in_old_script(4);
 #ifdef FEAT_FLOAT
     char_u	*p;
     int		get_float = FALSE;