Rename HA as a feature and in the docs

Overview

In gitlab-com/Product#1160 (closed) it was proposed that we rename GitLab High Availability, both as a feature in features.yml and in references throughout the documentation. The main argument for moving away from "High Availability" is that adding more complexity to a customer's architecture doesn't necessarily result in more availability of GitLab.

Proposal

The general consensus so far is as follows.

• "high availability" is replaced with "scaled" or "scaling" when referring more generally to a multi-node architecture
• "high availability" is replaced with "fault-tolerant" or "failover" when specifically referring to PostgreSQL See gitlab-com/Product#1160 (comment 349625500) for specific examples.

Other terms that were considered

• "Multi node" => we decided that this term doesn't adequately convey the user benefit of the feature or concept
• "PostgreSQL Cluster" => this naming was considered because Gitaly HA has been named "Gitaly Cluster". However, "PostgreSQL Cluster" already has a specific meaning in the PostgreSQL community.

Changes that need to be made

• features.yml @ljlane
• User documentation @axil
• https://about.gitlab.com/support/#priority-support => resolved by gitlab-com/www-gitlab-com!37448 (diffs)
• https://about.gitlab.com/pricing/self-managed/feature-comparison/ @ljlane

cc @gitlab-org/distribution @lyle @sunjungp for awareness