Lower go directive to 1.20 for Windows 7 users (#94)

go.mod

diff --git a/go.mod b/go.mod
index e7c8d90..49f8cc5 100644
--- a/go.mod
+++ b/go.mod
@@ -1,5 +1,5 @@
 module github.com/mattn/go-isatty
 
-go 1.21
+go 1.20
 
 require golang.org/x/sys v0.28.0