Increase CPU limit for migrations container

Context

We've had several alerts recently related to the CPU limits of the migration containers:

• https://console.cloud.google.com/monitoring/alerting/incidents/0.mw6ur8f0o2sa?project=gitlab-review-apps&pageState=(%22interval%22:(%22d%22:%22P1M%22,%22i%22:(%22s%22:%222023-04-13T09:35:49.000Z%22,%22e%22:%222023-04-13T11:46:42.000Z%22)))
• https://console.cloud.google.com/monitoring/alerting/incidents/0.mwm648zhy96o?project=gitlab-review-apps&pageState=(%22interval%22:(%22d%22:%22P7D%22,%22i%22:(%22s%22:%222023-04-24T11:29:44.000Z%22,%22e%22:%222023-04-24T19:17:46.000Z%22)))
• https://console.cloud.google.com/monitoring/alerting/incidents/0.mwmeub8cmo7i?project=gitlab-review-apps&pageState=(%22interval%22:(%22d%22:%22P1D%22,%22i%22:(%22s%22:%222023-04-24T17:51:45.000Z%22,%22e%22:%222023-04-24T20:17:47.000Z%22)))

What does this MR do?

Increase the CPU limit for migrations container, so that we don't receive those warnings.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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