Resolve "Change default size for GKE cluster creation"

What does this MR do?

This MR changes the default machine type for clusters created in CI/CD > Cluster.

• Old: n1-standard-4
• New: n1-standard-2

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Default to n1-standard-2 is enough.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39649 (closed)