[CC-5321] Add premium to Consul Cluster tier's description

[jjti]

🛠️ Description

This is just a docs change. This adds the premium tier to Consul Cluster: https://www.hashicorp.com/products/consul/pricing

It's already supported in the backing Go code as an enum:

	// HashicorpCloudConsul20210204ClusterConfigTierPREMIUM captures enum value "PREMIUM"
	HashicorpCloudConsul20210204ClusterConfigTierPREMIUM HashicorpCloudConsul20210204ClusterConfigTier = "PREMIUM"

JIRA: https://hashicorp.atlassian.net/browse/CC-5321

🏗️ 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=TestAccXXX'

...