Rename Praefect High Availability to Gitaly Cluster

What does this MR do?

High Availability is property of a system, and is confusing when used as the name of a feature. Particularly it is confusing given there are multiple aspects to making a GitLab instance HA.

Additionally, the work we are doing on Gitaly and Praefect is not exclusively for High Availability. A clustered configuration will provide performance improvements in the future.

• Removes Beta warning in preparation for GA in 13.0

Summary of a few names considered.

Name	Mutually Exclusive	Collectively Exhaustive	Few Words	Ubiquitous language
Gitaly High Availability	❌ High availability is used to refer to the configuration of lots of aspects of the GitLab application. Gitaly is also the name of a component.	❌ It does not not capture all use cases, like performance	✅ Short	✅ High Availability is ubiquitous
Praefect	❌ Would refer to both a component and a feature	✅ Covers performance, availability etc	✅ Short	❌ No one knows what Praefect is
Gitaly Cluster	⚠ Refers to a feature specific to Gitaly. Gitaly is also the name of a component.	✅ Covers performance, availability etc	✅ Short	✅ Clusters is a well understood term for the configuration that the feature enables

Related issues

gitlab-com/Product#1160 (closed)

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