Update to Go 1.16 and terraform-plugin-sdk 1.16

Author: joepjoosten

go.mod

@@ -1,10 +1,10 @@
 module github.com/gitlabhq/terraform-provider-gitlab
 
-go 1.15
+go 1.16
 
 require (
 	github.com/hashicorp/go-retryablehttp v0.6.8
-	github.com/hashicorp/terraform-plugin-sdk v1.15.0
+	github.com/hashicorp/terraform-plugin-sdk v1.16.0
 	github.com/mitchellh/hashstructure v1.0.0
 	github.com/xanzy/go-gitlab v0.44.0
 )