Docs: Update docs to note GitLab EE 11.10.x and above are not available via AWS Community AMIs

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Update docs to indicate and explain why GitLab EE versions 11.10.x are not available for installation via AWS Community AMIs.

• https://about.gitlab.com/handbook/alliances/cloud-images/#aws-image
• https://docs.gitlab.com/ee/install/aws/#choose-the-ami
• https://docs.gitlab.com/ee/university/high-availability/aws/#initiate-ami

Customer ticket: https://gitlab.zendesk.com/agent/tickets/123085 (internal)