bump required go version to 1.21

internal/backend/rest/rest_test.go

@@ -1,6 +1,3 @@
-//go:build go1.20
-// +build go1.20
-
 package rest_test
 
 import (

internal/backend/rest/rest_unix_test.go

@@ -1,5 +1,5 @@
-//go:build !windows && go1.20
-// +build !windows,go1.20
+//go:build !windows
+// +build !windows
 
 package rest_test