chore: remove sunset Go Report Card badge

Go Report Card has been sunset, so the badge in the README now renders as
'retired' and links to a shutdown notice. There is no successor service; the
go.dev reference badge already covers the same ground.

Fixes #1875

Author: james00012

README.md

@@ -1,7 +1,6 @@
 # Terratest
 
 [![Maintained by Gruntwork.io](https://img.shields.io/badge/maintained%20by-gruntwork.io-%235849a6.svg)](https://gruntwork.io/?ref=repo_terratest)
-[![Go Report Card](https://goreportcard.com/badge/github.com/gruntwork-io/terratest)](https://goreportcard.com/report/github.com/gruntwork-io/terratest)
 [![go.dev reference](https://img.shields.io/badge/go.dev-reference-007d9c?logo=go&logoColor=white&style=flat-square)](https://pkg.go.dev/mod/github.com/gruntwork-io/terratest?tab=overview)
 ![go.mod version](https://img.shields.io/github/go-mod/go-version/gruntwork-io/terratest)