Add old_syntax_only setting

This keeps the fidlbolt fidlc invocation in sync with the one
used to build in-tree, and will be necessary when SDK FIDL files
get updated to use `deprecated_syntax;`

Change-Id: Iae088d6acc22506a3f68557724111985bf0341e7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidlbolt/+/542866
Reviewed-by: Mitchell Kember <mkember@google.com>
diff --git a/backend/server.go b/backend/server.go
index 4369d2f..0fc5556 100644
--- a/backend/server.go
+++ b/backend/server.go
@@ -363,6 +363,7 @@
 			msg := fmt.Sprintf("%s:1:1: error: %s", fidl, err)
 			return response{Ok: false, Content: msg}, nil
 		}
+		args = append(args, "--experimental", "old_syntax_only")
 		run, err := s.fidlc.run(ctx, append(args, fileArgs...)...)
 		if err != nil {
 			return response{}, err