Increase cache timing for open issues count

What does this MR do?

Increase the cache timing for open issue count per user.

This is an expensive query that is invalidated easily, and the DB load is significant. It's an easy win by essentially offloading more to Redis (ie. it will remember the count for longer than perhaps necessary, but this can save a rather expensive DB hit, especially as opened issue count appears at the top right of every page layout)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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Related to #325470 (closed)