Follow-up: Add Gitlab::Redis::Sessions Redis instance

What does this MR do and why?

This MR simplifies the process of adding a new Redis health check. It also removes some duplicated code

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #1333 (closed)