Change dbInstanceType to computed for cluster resource

[vandyliu]

• [x] I have checked that a similar feature request does not already exist.

✍️ Is your feature request related to a problem? Please describe.

🧩 Describe the solution you'd like

• dbInstanceType should be computed and not required and not optional - the API can handle this field as optional

🤔 Describe alternatives you've considered

💡 Additional context