Upgrade go 1.18

[bcmdarroch]

🛠️ Description

Upgrades the provider to go version 1.18.

🚢 Release Note

Release note for CHANGELOG:

* provider: upgrade to 1.18 [GH-365]

🏗️ Acceptance tests

• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ make testacc

PASS
ok  	github.com/hashicorp/terraform-provider-hcp/internal/provider	5510.161s

Also ran a manual test with a Consul cluster, upgrading from the latest version to the provider build with these changes.

1. Initializing HVN + Consul cluster on 0.37.0:

$ terraform init
Initializing provider plugins...
- Finding hashicorp/hcp versions matching "~> 0.37.0"...
...

$ terraform apply
...
Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

2. Upgrading to local build with go upgrade:

$ terraform init
Initializing provider plugins...
- Finding localhost/providers/hcp versions matching "0.0.1"...
...

$ terraform apply
...
No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration and found no differences, so no changes are needed.

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.