Increase gitlab-shell memory request/limit

What does this MR do and why?

We recently had alerts about gitlab-shell reaching memory limits: https://console.cloud.google.com/monitoring/alerting/incidents/0.n44ehhoocy6z?project=gitlab-review-apps&pageState=(%22interval%22:(%22d%22:%22P1D%22,%22i%22:(%22s%22:%222023-11-02T11:45:01.000Z%22,%22e%22:%222023-11-02T12:41:57.000Z%22))).

MR acceptance checklist

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