Priority support for troubleshooting Gitaly cluster

What does this MR do?

Reminder that GitLab Support for scaled architecture (including Gitaly Cluster) is limited to only customers with Priority Support on the Gitaly Cluster page.

This can be confusing because there are two meanings of Support.

1. All tiers of the GitLab EE product (including Free/Starter) "supports/includes" Gitaly cluster in the code and functionality required to get Gitaly cluster working are avaialble to all tiers.
2. GitLab Support only offers professional tech support assistance to customers running Scaled architecture (including Gitaly cluster) for self-managed customers with Prioirty Support (Premium/Ultimate)

Slack thread in #spt_managers

Screenshots (strongly suggested)

https://about.gitlab.com/support/#priority-support

https://about.gitlab.com/support/#definition-of-scaled-architecture

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

CC @cat @lbot