Bump Go Version to 1.18 (#79)

Reference: https://github.com/hashicorp/terraform-provider-scaffolding-framework/issues/78

Updated via:

```shell
go mod edit -go=1.18
go mod tidy
go fix ./...
```

And textual file updates, such as the README.

README.md

@@ -17,7 +17,7 @@ Once you've written your provider, you'll want to [publish it on the Terraform R
 ## Requirements
 
 - [Terraform](https://www.terraform.io/downloads.html) >= 1.0
-- [Go](https://golang.org/doc/install) >= 1.17
+- [Go](https://golang.org/doc/install) >= 1.18
 
 ## Building The Provider