Add starter_small HCP Vault cluster tier

[jfreda]

🛠️ Description

This adds support for the newly launched starter_small HCP Vault cluster tier. The HCP SDK was bumped to support this change.

🚢 Release Note

Release note for CHANGELOG:

* resource/hcp_vault_cluster: add `starter_small` cluster tier

🏗️ Acceptance tests

• Are there any feature flags that are required to use this functionality?
• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccVaultCluster'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/... -v -run=TestAccVaultCluster -timeout 120m
?   	github.com/hashicorp/terraform-provider-hcp/internal/clients	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-hcp/internal/consul	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-hcp/internal/input	(cached) [no tests to run]
=== RUN   TestAccVaultCluster
--- PASS: TestAccVaultCluster (787.05s)
PASS
ok  	github.com/hashicorp/terraform-provider-hcp/internal/provider	787.583s

[bcmdarroch]

🎉