Adjust to best practices for Service Accounts on GCP

What does this MR do?

Google recommends to not rely on access scopes for permissions in VMs, nor on the default service account, so we follow their best practices detailed on https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances#best_practices

Related issues

These changes are required to resolve permissions issues in gitlab-com/gl-infra/ansible-workloads/db-provisioning!65 (merged)

Author's checklist

When ready for review, the Author applies the workflowready for review label:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up to date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: setup a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.