Revert go mod version to 1.20

This avoids new 1.21 go mod management behavior which automatically
upgrades a module's go mod version based on dependencies.

See mdlayher#13 for discussion

go.mod

@@ -1,6 +1,6 @@
 module github.com/mdlayher/socket
 
-go 1.21
+go 1.20
 
 require (
 	github.com/google/go-cmp v0.5.9